Measurement probe

ABSTRACT

An ultrasound probe is described that comprises a transducer for transmitting and receiving ultrasound. The probe also includes a coupling element, such as a spherical ball of self-lubricating or hydrogel material, for contacting and acoustically coupling to an object to be inspected. The ultrasound probe also includes an analyser that is arranged to analyse the ultrasound signal received by the transducer and thereby determine if there is contact between the coupling element, and the surface of an object. The probe can thus be used for internal (ultrasound) inspection of objects as well as measuring the position of points on the surface of the object. The probe may be mountable to a coordinate measuring machine or other moveable platforms.

The present invention relates to ultrasound inspection apparatus and inparticular to an ultrasound probe that can acquire both ultrasound andsurface contact measurements of an object.

It is known to measure the dimensions of manufactured objects to ensurethey conform to tolerance. In the case of high value components, such asaerospace turbine blades, the external form of an object can be measuredto sub-micron accuracy using a surface contact probe mounted on a CMM.Examples of techniques for measuring the positions of multiple points onthe surface of an object using a CMM equipped with a surface contact(e.g. scanning) probe are described in U.S. Pat. No. 5,189,806 andWO2009/024783.

In addition to surface measurements, it is often necessary to measurethe internal features of an object. For example, turbine blades aretypically hollow to enable them to be both light-weight and strong foroperation at extreme temperatures and pressures. The internal inspectionof such hollow turbine blades is typically carried out using ultrasoundinspection. Ultrasound thickness gauging probes, which typically rely onthe localised application of a couplant material (e.g. a coupling gel orliquid) to the part, are known. Such probes tend to be handheld andprovide thickness measurements when placed into contact with a point onthe surface of the object being inspected. It has been describedpreviously how an ultrasound probe may be mounted to the quill of a CMM.For example, US2009/0178482 describes an ultrasound probe mounted to thequill of a CMM. US2009/0178482 also describes how the CMM mayinterchangeably use the ultrasound probe and a standard surface contactprobe for internal and external (surface position) measurementsrespectively.

According to a first aspect of the present invention, there is providedan ultrasound probe comprising;

-   -   a transducer for transmitting and receiving ultrasound, and    -   a coupling element for contacting and acoustically coupling to        an object to be inspected,    -   wherein the ultrasound probe comprises an analyser arranged to        analyse the ultrasound signal received by the transducer and        thereby determine if there is contact between the coupling        element and the surface of an object.

The present invention thus relates to an ultrasound probe that includesa transducer for transmitting and receiving ultrasound. The ultrasoundtransducer may, for example, comprise a pulse-echo ultrasonic transducerthat includes a piezo-electric element for transmitting high frequencytime-discrete longitudinal waveforms (hereinafter termed “L-waves”). Theultrasound probe also includes a coupling element (e.g. a hydrophilicsphere or other tip as described below) that is designed to contact andacoustically couple with the object to be inspected. An analyser is alsoprovided to analyse the ultrasound signal received by the transducer(e.g. the echoes returned following an excitation pulse) and use thatreceived signed to establish if there is contact between the couplingelement and the surface of an object. The ultrasound probe of thepresent invention is particularly advantageous when mounted tocoordinate positioning apparatus because it permits the position ofpoints on the surface of an object to be measured.

The present invention thus provides an ultrasound probe that not onlyallows the internal features (e.g. thickness) of objects to be measuredbut also determines when the probe contacts an object or is moved out ofcontact with an object. This allows the probe to be used for boththickness and surface position measurements. In other words, theultrasound probe of the present invention removes the need to use aseparate surface contact (e.g. touch trigger) measurement probe incombination with an ultrasound probe. The ultrasound probe of thepresent invention can perform both internal and external tasks, therebyincreasing the speed of part inspection.

The transducer may be configured to operate in any suitable mode. Theultrasound probe may operate at a high frequency. For example, theoperating frequency may be greater than 5 MHz, greater than 10 MHz ormore preferably greater than 15 MHZ. In a preferred embodiment, theoperating frequency is around 20 MHz. The transducer, which may comprisea piezoelectric element, preferably excites longitudinal sounds waves(L-waves). Advantageously, the transducer is arranged to periodicallytransmit an excitation pulse and subsequently receive the returnedultrasound signal (i.e. pulse-echo mode operation). The amplitude of theultrasound signal received after each excitation pulse may then bemeasured as a function of time to provide an amplitude scan (also termedan A-scan). A plurality of such scans may be combined and averaged, ifrequired. The analyser may determine if there is contact between thecoupling element and an object from differences in successive A-scans.

Advantageously, each amplitude scan includes amplitude peaks relating tointernal reflections of ultrasound from within the probe. The internalreflections may arise from a delay line, the coupling element or otherinterfaces between different type of material within the probe. Theanalyser may determine if there is contact between the coupling elementand an object from changes in the amplitudes of the internal reflectionsbetween successive amplitude scans. Alternatively, the analyser maydetermine if there is contact between the coupling element and theobject by assessing if internal reflections from a contacted object arepresent in each amplitude scan. For example, the appearance ofadditional reflection peaks may be used to indicate the existence ofback wall reflections from an object and hence that contact with anobject has been established. In addition to determining if contact withan object has been established, it is also possible to determine whencontact with an object has been lost in a similar manner.

Conveniently, the ultrasound probe outputs a trigger signal thatindicates when the analyser has determined the coupling element is incontact with an object. In other words, the ultrasound probe may issue atrigger signal when an object is contacted. For example, a triggersignal output line may be held high (status “1”) to indicate when thereis contact with an object and held low (status “0”) to indicate noobject contact. The transition from low to high thus indicates contacthas been made, and high to low indicates contact has been lost. Such atrigger signal may be fed to the controller of a coordinate positioningapparatus and used to determine the position of points on the surface ofan object in a similar manner to measurements taken with a traditionaltouch trigger probe.

In addition to determining when contact with an object is lost orestablished, the ultrasound probe may also determine when optimumacoustic coupling with an object is obtained. In particular, theanalyser may assess when the “coupling sweet spot” is attained bymonitoring reflections from the object as the probe is pressed into theobject with increasing or decreasing force. In particular, the analyserpreferably determines when optimum ultrasound coupling with an object isachieved by monitoring internal ultrasound reflections from thecontacted object as the coupling element is moved into closer engagementwith the object. Once the “coupling sweet spot” is attained, ultrasoundmeasurements (e.g. thickness measurements, crack detections measurementsetc) may then be acquired.

The ultrasound probe can also probe the internal structure of the objectusing the ultrasound that is coupled into the object. Advantageously,the analyser determines the thickness of a contacted object frominternal ultrasound reflections from the contacted object. Preferably,the analyser assesses the thickness of a contacted object fromsuccessive reflections from the back wall of the object. In other words,the probe may measure thickness using the so-called Mode-3 operationthat is described in more detail below. It would, of course, be possibleto use different modes of operation (e.g. Mode-1 or Mode 2) for theultrasound inspection. The probe may thus be configured to operate inany one of a plurality of different ultrasound measurement modes. Thethickness measurements may require a calibration step to be performed(e.g. to measure the speed of sound in the material of the part beinginspected). The ultrasound probe may comprise a probe housing in whichthe transducer and coupling element are provided. The analyser may beprovided within the probe housing or in an interface external to theprobe housing.

Advantageously, the coupling element comprises an elastically deformablematerial. If internal reflections from the coupling element are beingmonitored, deformations of the coupling element alter such reflections.Elastic deformability also means the coupling element returns to thesame shape after a distorting force is removed. Such elasticity is thusbeneficial as it allows repeatable and accurate touch contactmeasurements to be made. It is also preferable for the coupling elementto be substantially spherical as surface contact measurements are thenhighly repeatable.

The coupling element of the probe may comprise a dry couplant material,such as silicone rubber or the like. The coupling element may requirethe application of a coupling material (e.g. water or gel) to ensuregood acoustic coupling. The coupling element could comprise acompressible and easily pliable oil-based thermo-softening plastic withlow acoustic attenuation properties.

Advantageously, the coupling element comprises a self-lubricatingmaterial. Such a self-lubricating material preferably releases alubricant, such as water and/or oil, from its external surface in acontrolled manner. The self-lubricating material may comprise anoleophilic elastomer. Advantageously, the self-lubricating materialcomprises a hydrophilic elastomer. For example, the hydrophilicelastomer may comprise a incompressible gelatinous hydrophilic elastomermaterial such as a lightly cross-linked hydrophilic vinyl elastomer or asuper absorbent polymer hydrogel. An example of a high water contenthydrophilic polymer chain compounds is MMA:VP (i.e. a Copolymer ofN-vinyl pyrrolidone and methyl methacylate). For this compound, thewater content can vary from about 35% to 95% and excellent acousticproperties are exhibited although the tear strength decreases as thewater content is increased. Conveniently, the self-lubricating materialis provided as a sphere. A preferred embodiment thus comprises acoupling element comprising a hydrophilic elastomer sphere.

The provision of a coupling element comprising a hydrophilic elastomersphere has a number of advantages. For example, water swellinghydrophilic polymer spheres release a limited amount of water from theirsurface (i.e. they “sweat” water). This released water provides improvedacoustic coupling with an object by filling any pockets of air betweenthe ultrasound probe and a rough surface of that object. The amount ofwater released from the hydrophilic elastomer sphere can be controlledby appropriate selection of the chemical properties of the polymermaterial. For example, the amount of released water can be arranged tobe minimal and will thus evaporate quite readily in the atmospherewithout leaving any residual contaminant. Furthermore, such hydrophilicelastomer spheres can be soft and elastic to provide a high degree ofconformity against a curved inspection surface. The release of the watercan also act as a lubricant that permits such spheres to be scannedalong a path on the surface of an object. Furthermore, hydrophilicmaterials have inherently low acoustic attenuation and an acousticimpedance value that is well suited to ultrasonic transmission between aceramic transducer wear plate and metallic part.

Advantageously, the ultrasound probe comprises a delay line. The probemay include a coupling element that also acts as a delay line. Forexample, a coupling element may be provided that comprises a hydrophilicelastomer sphere that has a protruding portion for contacting the objectto be inspected and is sufficiently large to provide a delay linefunction. Conveniently, the probe comprises a delay line that is coupledto the coupling element. In other words, a delay line (e.g. a solidplastic delay line) that is separate to the coupling element may beprovided. Such a delay line may, for example, be formed from Polystyreneor Polycarbonate. A hydrophilic elastomer sphere may be coupled to thedistal end of the delay line. The delay line may thus be placed in theacoustic path between the transducer and the coupling element.

Advantageously, the ultrasound probe comprises an ultrasound beamcontrol element. The ultrasound beam control element preferablymanipulates (e.g. refracts, steers or focuses) the ultrasonic wavefrontthat is transmitted into, and/or is received from, the object beinginspected. Preferably, a delay line is provided that also acts as theultrasound beam control element. For example, the ultrasound probe maycomprise a tapered delay line that results in a more divergent far-fieldbeam. An ultrasound beam control element may also be provided in theform of an acoustic lens. For example, a spherically focussed planarconcave lens could be provided. A hydrophilic elastomer sphere couldthen be coupled to the concave lens (e.g. so the lens cups around thesphere) to provide refractive focussing to increase back wallreflections. An ultrasound beam control element may be provided in theform of a refractive wedge. For example, an asymmetric rigid wedge couldbe provided that refracts the projected ultrasonic beam into theinspection surface at some set angle from the normal direction asdetermined by the relative speed of sound in the coupled material. Thiscan be useful for internal metrology measurements of more complexgeometries where the front wall and back wall are not parallel. Acousticmirrors and other acoustic components may be included in the probe, asrequired.

Advantageously, the ultrasound probe comprises an ultrasound absorbingshell. The ultrasound absorbing shell suppresses or attenuates allunwanted acoustic reflections from within the shell walls that mayotherwise interfere with the reflected waveform of interest. Theultrasound absorbing shell may be formed from, for example, Teflon® orglass-filled PTFE.

The ultrasound probe may be provided as unitary item. Advantageously,the ultrasound probe is a modular probe. The probe thus preferablycomprises a base module that comprises the transducer and a couplingmodule that comprises the coupling element. The base module may comprisea first connector portion and the coupling module a second connectorportion. The first connector portion is preferably releasably attachableto the second connector portion. The base module of the probe may beattached, or attachable, to the moveable member of a coordinatepositioning apparatus. For example, the base module may include anattachment feature or mechanism that permits it to be attached to thequill, or rotary head, of a CMM.

The ultrasound probe may thus form part of modular ultrasound inspectionapparatus that includes a base module and a plurality of couplingmodules. This allows different coupling modules to be attached to thebase module as and when required. The attachment and detachment ofcoupling modules is preferably performed in an automated manner byappropriate programming of the coordinate positioning apparatus. Theplurality of coupling modules may include a range of different couplingmodules (e.g. that direct sound in different directions and/or withdifferent amounts of divergence) for measuring different internalfeatures of a part. The plurality of coupling modules may alternativelyor additionally include a range of similar coupling modules that have alimited lifetime (e.g. due to wear or damage of a soft coupling element)and can thus be replaced when damaged, worn or after being used for acertain period of time. The plurality of coupling modules may thus beconsumable items that have a short lifetime compared with the basemodule.

Advantageously, a holder (e.g. a storage tray) is provided for retainingthe plurality of coupling modules. The holder may be attachable to thebed of coordinate positioning apparatus. For example, the holder mayinclude one or more features (e.g. screws holes, magnets etc) that allowthe holder to be secured to the bed of a CMM in a fixed position andorientation. The holder may include one or more ports or receptacles,each port or receptacle arranged to retain a coupling module. The holdercan thus store the coupling modules that are not presently being used;i.e. coupling modules that are not attached to the base module formeasurement purposes can be stored in the holder. The holder may storemore than five, more than ten or more than fifteen coupling modules. Theapparatus may thus comprise more than five, more than ten or more thanfifteen coupling modules. The plurality of coupling modules may comprisea plurality of different designs or types of coupling modules. Theplurality of coupling modules may comprise a plurality of substantiallyidentical coupling modules.

As explained above, contact modules can be provided in which thecoupling element is self-lubricating (e.g. slowly releases water). Theholder may thus be hermetically sealed prior to use. This prevents thecoupling elements (e.g. hydrophilic spheres) from drying out prior touse. The holder may be re-sealable. For example, the holder may beopened and closed after each coupling module is taken or it may beopened and then closed after a set of measurements have been acquired.Alternatively, the holder (and optionally the coupling modules havingself-lubricating coupling elements) may be provided as a single-use orconsumable item that is opened, used and then discarded or recycled. Theholder may initially contain coupling modules comprising de-hydratedhydrophilic spheres that are hydrated prior to use. A new holder maythus be opened as and when required.

In a preferred embodiment, the holder comprises a plurality of recessesfor receiving the plurality of coupling modules. Conveniently, therecesses and coupling modules are arranged to prevent rotation of acoupling module when located in a recess. For example, the couplingmodules may include a central hub with one or more radially protrudingwings. The holder may then comprise a complimentary recess. A couplingmodule may thus be placed into and withdrawn from a recess usingvertical (linear) relative motion. Once inserted, rotation of thecoupling module relative to the holder is prevented.

The first and second connector portions may be provide by any mechanicallinkage. For example, a magnetic connection arrangement may be provided.Advantageously, the first and second connector portions comprisecomplementary screw threads. For example, the first connector portion ofthe base module may comprise a screw thread provided on the outersurface of the distal end of the base module housing (e.g. a malescrew-thread connector). The second connector portions of the couplingmodules may then include a recess with an internally screw threadedsurface (e.g. a female screw-thread connector). The arrangement may besuch that the base module and coupling module can engage and disengagefrom each other by imparting relative rotary motion. This allows a basemodule (e.g. held by a rotary head) to be screwed into and out ofengagement with a coupling module. The coupling modules may convenientlybe stored in a holder of the type described above that prevent rotationduring the attachment process.

Attachment of one of the plurality of coupling modules to the basemodule preferably establishes a reliable and repeatable acoustic linkagebetween the modules. In particular, mating the first and secondconnector portions provides an acoustic link between the transducer ofthe base module and the coupling element of the coupling module. Thebase module thus preferably comprises a transducer having a wear plate.The wear plate is conveniently arranged to acoustically couple with acoupling module attached to the base module. For example, the wear platemay engage the delay line of a coupling module or it may directly engagea hydrophilic elastomer sphere of a coupling module. In such anembodiment, attachment of one of the plurality of coupling modules tothe base module holds the relevant part of the coupling module (e.g.delay line, sphere etc) firmly against the transducer wear plate.

The present invention also extends to a coordinate positioning apparatusthat includes the above described ultrasound probe. The ultrasound probemay be mountable, or mounted, to the moveable member of the coordinatepositioning apparatus. The coordinate positioning apparatus may includea machine tool, an industrial robot, an arm, an x-y scanner or acrawler. In a preferred embodiment, the coordinate positioning apparatuscomprises a coordinate measuring machine. The CMM may be a Cartesian(e.g. bridge type) of CMM or a non-Cartesian (e.g. hexapod) type of CMM.The CMM preferably comprises a rotary head that provides the moveablemember to which the probe is attached. The rotary head may comprise asingle rotary axis, two rotary axes or three rotary axes. The rotaryhead advantageously comprises at least two rotary axes. The rotary headmay comprise at least three rotary axes.

According to a further aspect of the invention, there is described amethod of operating a pulse-echo ultrasound probe mounted to coordinatepositioning apparatus to acquire surface position measurements. Themethod comprises the step of monitoring the echoes of receivedultrasound signals for changes indicative of contact with an object. Anultrasound probe may thus be adapted to also acquire surface contactmeasurements.

According to a further aspect of the invention, an ultrasound inspectiondevice for coordinate positioning apparatus is provided. The devicecomprises an ultrasound transducer and a coupling element for contactingand acoustically coupling to an object to be inspected, wherein thecoupling element comprises self-lubricating material. The speed of soundwithin the coupling element may be measured by analysis of reflectionsfrom within the coupling element when the coupling element is subject toa plurality of different deformations. The ultrasound inspection devicemay be a modular design, as described above, or a single (unitary ornon-modular) arrangement. The device may comprise any of the featuresmentioned herein. Advantageously, the self-lubricating materialcomprises a hydrophilic elastomer. Conveniently, the self-lubricatingmaterial comprises a super absorbent polymer hydrogel. Preferably, thecoupling element comprises a sphere of self-lubricating material. Theultrasound inspection device may be used in a method of measuring theporosity and/or density of a part, such as a part made using an additivemanufacturing technique.

The present invention also extends to a method of measuring thethickness of an object, and/or points on the surface of an object, usingapparatus described above.

The invention will now be described, by way of example only, withreference to the accompanying drawings in which;

FIGS. 1a-1c illustrate various known ultrasound transducer arrangements,

FIG. 2 illustrates the principle of ultrasound thickness measurement,

FIG. 3 shows a modular ultrasound inspection apparatus mounted on a CMM,

FIGS. 4(a) and 4(b) illustrate straight and cranked versions of themodular ultrasound probe,

FIGS. 5(a) to 5(c) show a dispensing tray and a plurality of associatedcoupling modules,

FIG. 6 shows in more detail an ultrasound probe comprising a couplingmodule with a tip comprising a hydrophilic elastomer sphere,

FIG. 7 shows the ultrasound probe with a coupling module that providebeam projection normal to the surface of the object being inspected,

FIG. 8 shows the ultrasound probe with spherical focussing,

FIGS. 9(a)-9(c) are images of the coupling module of the modularultrasound probe,

FIGS. 10(a) and 10(b) are images of a coupling module attached to a basemodule comprising an elongate carbon fibre tube,

FIG. 11 is an example of a transmit/receive circuit for driving themodular ultrasound probe,

FIG. 12 illustrates steps for generating an ultrasound inspection partprogram,

FIG. 13 illustrates an ultrasound calibration block,

FIG. 14 shows a cranked variant of the modular ultrasound probe scanninga turbine blade,

FIGS. 15(a) to 15(d) illustrate the A-scan waveforms produced duringthickness measurement,

FIG. 16 shows the modular ultrasound probe with a hydrophilic sphericaltip being brought into contact with the surface of an object,

FIG. 17 shows the deformation of the spherical tip that occurs duringthe surface contact shown in FIG. 16,

FIG. 18 shows the variation of positional information as the sphericaltip is moved into the objects surface,

FIG. 19 shows the effect of tilt on the sphere centre position,

FIGS. 20(a) and 20(b) show sphere displacement as a function ofultrasound reflection time of arrival data,

FIG. 21 illustrates using the modular ultrasound probe to scan anundulating surface,

FIGS. 22(a) to 22(c) show probe loading scenarios for different parts tobe inspected,

FIG. 23 shows a modular ultrasound probe configured to measure thebottom surface of a straight bore,

FIG. 24 shows a modular ultrasound probe configured to measure thebottom surface of an angled bore,

FIG. 25 shows a modular ultrasound probe configured to measure the sidewalls of a bore,

FIG. 26 shows examples of the ultrasonic waveforms measured during aninspection using an ultrasound probe with a rubber tip,

FIG. 27 shows calibration plots of z-axis displacement of the couplingmodule tip versus reflection peak amplitude attenuation,

FIG. 28 is a block diagram illustrating the basic principle of a methodfor estimating acoustic delay,

FIG. 29 illustrates operation of the phase transform replica correlationalgorithm that can be employed for improved accuracy in estimatingtime-delays,

FIG. 30 is a flow chart illustrating the steps of a combined surfacepoint and thickness measurement method,

FIG. 31 illustrates use of the modular ultrasound probe on an XYscanner, and

FIG. 32 illustrates use of the modular ultrasound probe on aself-contained crawler.

Referring to FIGS. 1a to 1 c, a variety of ultrasound probes thatcomprise longitudinal wave (L-wave) transducers for internal metrologymeasurements are shown. Such probes have been used previously forinspection purposes, typically as handheld inspection devices.

FIG. 1a shows an ultrasound probe 2 that comprises an outer body 4. AnL-wave transducer is provided that comprises an active piezoelectricelement 6. The relatively thin piezoelectric element 6 is arranged tohave a thickness approximately equal to half the wavelength ofultrasound being generated; this allows the high frequency excitationthat is required for accurate rise-time excitation. The piezoelectricelement 6 is backed by a thick attenuating backing material 8 to absorbenergy from the piezoelectric element 6 and thereby generate the desiredheavily damped response in the forward direction. This provides optimalrange resolution.

A delay line 10 is acoustically coupled to the piezoelectric element 6via a wear plate 12. The wear plate 12 protects the piezoelectricelement 6. The wear plate 12 has a thickness equal to one quarter of theultrasound wavelength to enable it to act as a matching layer; this wearplate thickness is preferred because it ensures the waves generated inthe piezoelectric element 6 are in phase with those reverberating withinthe wear plate 12. This means the amplitude of the ultrasound waveswithin the wear plate 12 and piezoelectric element 6 are additive andmaximum energy therefore enters the delay line 10 that is coupled to thewear plate 12. A liquid couplant layer (not shown) is provided at thedistal end of the delay line 10 to provide acoustic coupling with theobject 14 being inspected. In the example shown in figure la, the delayline 10 comprises a tapered propagation medium. The propagation mediummay be a poly-carbonate resin or cross-linked polystyrene. Fine axialgrooves 16 are machined into the sides of the propagation medium inorder to suppress internal reflections from within the propagationmedium.

FIG. 1b shows an ultrasound probe 20 that has many common features tothe probe 2 described above with reference to FIG. 1. The probe 20,however, has a non-tapered delay line 22 that has circumferentialcorrugation features 24.

The main function of a delay line, such as the delay lines 10 and 22described with reference to FIGS. 1a and 1 b, is to physically removethe ultrasonic excitation far enough away from the inspection surface totemporally resolve the excitation response from the initial reflectionfrom the back wall 26 of the object 14 being inspected. Preferably, thisis achieved without any temporal interference from the finite bandwidthexcitation pulse ring-down. It can thus be seen that delay lines performthe function of controlling when the ultrasound (i.e. the longitudinalwave) enters the inspection part. Different degrees of tapering can beemployed to generate higher contact pressures and accommodate morecurved parts. Such tapering also affects the natural focal length andbeam divergence (i.e. a diffractive effect) of the device.

Ultrasound probes as described with reference to FIGS. 1(a) and 1(b),provide normal incidence, unfocussed (in the near field) or naturallydivergent beam inspections. It is also possible to project ultrasoundwaves at angles away from the surface normal (e.g. in internal defectdetection and sizing). FIG. 1(c) shows an alternative ultrasound probethat comprises a refractive angle beam wedge 30 (which is sometimesreferred to as an ultrasonic shoe). The beam wedge 30 is coupled to thepiezoelectric element 6 via a wear plate 12 as per the examplesdescribed with reference to FIGS. 1a and 1 b. The beam wedge 30 projectsthe ultrasonic waveforms at an off-axis beam angle from the surfacenormal of the object being inspected. For such wedge transducers,refraction at the interface between materials of different acousticimpedance is in accordance with the law of refraction (i.e. Snell's Law)and generation of a following shear wave mode (S-wave) at the interfaceby the phenomenon of mode conversion.

FIG. 1(c) also shows ultrasound projected from the beam wedge 30 of theultrasound probe into a metallic part 32. A slower S-wave refracts lessfrom the surface normal N than the faster L-wave mode. Moreover, it ishighlighted that the relative proportion of refracted L-wave andfollowing S-wave depends primarily upon the angle of incidence with theS-wave. It is also noted that a significant reflected mode is generatedat the interface (i.e. an R-wave is generated) that re-directs spuriousacoustic energy within the wedge. An absorbent shell 34 is thus coupledaround the propagating wedge material to attenuate this reflectedenergy. This shell 34 prevents the reflections that would otherwiserebound within the beam wedge 30 and interfere with the reflections ofinterest from the coupled metallic part 32.

It should be remembered that the ultrasound probes described above withreference to FIGS. 1(a) to 1(c) are examples and different designs ofultrasonic delay lines, wedges and lenses have been developed previouslyto allow optimal acoustic coupling against different solid inspectionparts. In the examples outlined above with reference to FIGS. 1(a) to1(c), it is described how an additional coupling layer (e.g. a layer gelor grease) is provided between the ultrasound probe and the object beinginspected. This is because all real inspection parts will exhibit somesurface asperities within the micro-structure that cause air-pockets tobe trapped at the interface between the probe and the inspectionsurface. The presence of such air pockets degrades acoustic couplingefficiency, primarily due to the large impedance mismatch between solidsand air. The use of a coupling layer, such as a gel, can thus ensure thenecessary acoustic coupling efficiency is obtained.

In cases where liquid water or gel cannot be liberally applied to theobject being inspected (e.g. in automotive applications), couplinglayers in the form of dry-coupling solids have been used previously.Several hydrophilic elastomers (e.g. Aqualene from Olympus) andsilicone-rubber based materials (e.g. Ultracouple from Sonemat) arecommercially available for dry-coupling ultrasonic non-destructivetesting (NDT) applications. However, their coupling performance is notalways suited to higher frequency precision thickness measurement probesdue to increased L-wave attenuation at the higher probe operatingfrequency (15-20 MHz). The excessive material stiffness of suchmaterials can also limit close conformity to more curved inspectionsurfaces. Moreover, silicone based couplant materials are considered tobe an unacceptable contaminant within some manufacturing environments(e.g. aerospace).

Referring to FIGS. 2(a) and 2(b), the basic principle of operation of apulse-echo thickness measurement transducer is outlined. In particular,FIG. 2(a) shows an ultrasound probe 40 similar to the ultrasound probe20 described above with reference to FIG. 1(b). The ultrasound probe 40includes a single-element ultrasonic delay line 22 that comprises adry-coupling polymer pad 42 for coupling to the object 14 to beinspected.

FIG. 2(b) is an example of the acoustic waveform received by thetransducer's active piezoelectric element 6 in response to a transienthigh voltage excitation pulse being applied across the piezoelectricelement 6. This time-domain waveform, referred to as an “A-scan” plot,is more often a time averaged response from a train of such excitationpulses (e.g. a sequence of N pulses where N may be in the range of16-32) in order to suppress random uncorrelated electronic noise.

The initial excitation pulse generated by the piezoelectric element 6 islabelled as the “Tx-Pulse” in FIG. 2(b). This initial excitation pulsecauses the inspection L-wave to propagate into the delay line 22 andtravel along the delay line at the speed of sound (C_(l)). The firstreflection peak (DL1) received back at the piezoelectric element 6arises from reflection of sound from the distal end of the delay line(i.e. from the interface between the delay line 22 and the polymer pad42). This reflection from the distal end of the delay line interface(i.e. the DL1 pulse) can be seen to occur at a time well after theinitial transmit pulse (Tx-pulse) has completely receded.

Although most of acoustic energy is reflected back at the delay lineinterface and never enters the inspection part, a sufficient proportionof acoustic energy does transmit into the coupled part 14 as ameasurable inspection pulse from which subsequent thickness measurementscan be made. Due to the acoustic impedance mismatch between the metallicpart 14 (e.g. Z ˜46 MRayl) and surrounding air (i.e. Z=0.000429 MRayl),the inspection L-wave propagates very efficiently within the part withonly very modest attenuation incurred from acoustic leakage acrossseveral reflections at the back wall interface. Since the speed of soundwithin the delay line 22 is low compared to the speed of sound withinthe thin metallic inspection part 14, multiple reflections between theback wall of the part 14 can occur before a second reflection peak (DL2)from the delay line is registered at the transducer. These back wallreflections thus provide the pulses BW1, BW2, and BW3 that can also beseen in the “A-scan” plot of FIG. 2(b). The time window observed withinthe A-scan between the first and second delay line reflection peak (DL1and DL2) is thus the probe's primary measurement window.

The thickness of the part 14 may be calculated from A-scan data of thetype shown in FIG. 2(b) in several ways. In practice, such thicknessmeasurements typically involve one of three modes of operation fromwhich the time delay information can be extracted from the measuredA-scan. These different modes are typically termed Mode-1, Mode-2 andMode-3 respectively. In Mode-1 gauging, the time delay measurement ismade between the excitation pulse (t=0) and the first back-wallreflection or primary echo from the inspection part. Mode-1 is usuallyassociated with direct contact transducers. In Mode-2 gauging, the timedelay measurement is made between an interface echo representing thenear surface of the test part and the first back wall reflection. Mode-2is typically used with a delay line or immersion transducer. In Mode-3gauging, the time delay measurement is made between two or moresuccessive back wall reflections. Mode-3 is typically used with a delayline or immersion transducer. Mode-3 is most effective where clean highSNR multiple back wall echoes can be observed, suggesting it is mostpractical in low attenuation high acoustic impedance parts such asfine-grain metals, glass or ceramics. Mode-3 also has the advantage thatit does not rely on the absolute time of arrival of back wall or delayline reflections, thus negating the effects of variability in thecoupling and delay lines of different coupling modules. Mode-3 alsoallows parts to be measured that comprise outer coating layers. Anysuitable mode (e.g. Mode-1, Mode-2 or Mode-3) could be used asnecessary. The use of different modes, possibly with different couplingmodules, may also be implemented during a measurement process.

Referring to FIG. 3, modular ultrasound inspection apparatus of thepresent invention is illustrated mounted on a coordinate measuringmachine 50. The CMM 50 comprises a quill 52 that can be moved alongthree mutually orthogonal linear axes (X, Y and Z). A two-axis rotaryhead 54, such as the REVO® active head produced by Renishaw plc, ismounted to the quill 52 of the CMM 50. A modular ultrasound probe 56 isin turn carried by the rotary head 54. The ultrasound probe 56, which isshown in an expanded view in the inset to FIG. 3, comprises a basemodule 58 and an attached coupling module 60. The base module 58 isattached to the rotary head 54 by a standard probe joint that allows theultrasound probe 56 to be attached to, and detached from, the two-axisrotary head 54 as required. Additional probes, for example, aconventional surface contact (scanning) probe 72 with a ruby tippedstylus can be stored in a probe rack 74 for exchange with the ultrasoundprobe 56. A calibration artefact 76 is also provided on the CMM bed andin this example a turbine blade 62 held by fixture 64 provides theobject to be measured.

As will be explained in more detail below, the ultrasound probe 56 has amodular arrangement. The base module 58 comprises a piezo-electrictransducer and wear plate, whilst the coupling module 60 comprises anacoustic delay line and a coupling element for contacting the object tobe measured. The modular ultrasound probe 56 is illustrated in FIG. 3with the coupling module 60 attached to the base module 58. Theultrasound inspection apparatus also comprises a plurality of additionalcoupling modules 66 that are retained in a storage tray 68 that isplaced on the bed of the CMM. In use, any one of the additional couplingmodules 66 may be exchanged with the coupling module 60. In other words,any of the additional coupling modules 66 can be attached to the basemodule 58 and used for measuring internal properties of an object. Theprocess of exchanging the coupling module attached to the base module 58is performed in an automated manner. For example, a magnet basedconnection or a screw-thread connection may be employed. The CMM 50comprises a computer 70 that controls CMM operation and also controlsthe automatic exchange of the coupling modules.

FIGS. 4(a) to 4(d) show two examples of modular ultrasound probes thatmay be used with the CMM described above with reference to FIG. 3.

FIGS. 4(a) and 4(b) show the modular ultrasound probe 56 that wasschematically illustrated in FIG. 3. The probe 56 comprises a basemodule 58 having a proximal end 90 that can be attached to the rotaryhead 54 of the CMM 50 via a standard probe joint. The base module 58also comprises an elongate shaft 94 and includes a piezoelectrictransducer 92 having a wear plate 93 located near the distal end of theelongate shaft 94.

FIG. 4(b) provides an expanded view of the distal end 96 of the modularultrasound probe 56 shown in FIG. 4(a). The distal end of the elongateshaft 94 can be seen to also comprise a first connector portion 98. Thecoupling module 60 includes a second connector portion 100. The firstconnector portion 98 and the second connector portion 100 are arrangedto allow attachment (and subsequent detachment) of the coupling module60 and base module 58. In other words, the first connector portion 98and the second connector portion 100 are complementary connectors thatcan be releasably linked together. As explained below, this connectionmay be achieved using a screw thread arrangement or in a variety ofalternative ways (e.g. via magnetic linkages etc). The coupling module60 includes a delay line 102 and a tip 104 for contacting the object tobe measured. Attachment of the coupling module 60 to the base module 58via the first connector portion 98 and the second connector portion 100causes the delay line 102 to engage the wear plate 93 thereby allowingultrasound to be coupled from the piezoelectric transducer 92 into thedelay line 102 and then into the object via the tip 104.

FIGS. 4(c) and 4(d) show a variant of the above described modularultrasound probe 56. Instead of the base module including asubstantially straight elongate shaft 94, the modular ultrasound probe109 comprises a cranked shaft 110. This provides a different angularorientation of the tip 104 of the coupling module 60 relative to therotary head 54 and is advantageous for certain inspection processes. Thecranked modular ultrasound probe 109 may be stored in a CMM rack (e.g.rack 74 described with reference to FIG. 3) and used instead of thenon-cranked modular ultrasound probe 56, as required.

The provision of a modular ultrasound probe as described herein has theadvantage that a range of different coupling modules may be attached tothe base module. These coupling modules may, for example, provide arange of different coupling properties that can be used for differentultrasound measurements. FIGS. 5(a) to 5(c) show an example of howmultiple coupling modules could be constructed and stored.

FIG. 5(a) shows in more detail the storage tray 68 described above withreference to FIG. 3. The storage tray 68 comprises a five-by-five arrayof storage slots 142 (although any arrangement or number of slots couldbe provided). Each of the storage slots 142 comprises a central holewith two radially extending slots. FIG. 5(b) shows the winged outershell 144 of a coupling module that may be placed into and retained byany one of the storage slots 142. An internal surface of the couplingmodule comprises a screw thread 146 to provide a connector portion thatcan be screwed into engagement with a complementary screw thread of thebase module of the ultrasound probe (not shown). The complementaryshapes of the storage slots 142 and outer shell 144 restricttranslational movement of any inserted coupling module in the plane ofthe tray (e.g. XY-plane). Moreover, the wings provided on the outershell 144 allow the coupling modules to be freely inserted into andextracted from the storage tray 68 in a direction normal to the plane ofthe storage tray 68 (e.g. via Z-direction movements of the CMM quill)but restrict any rotational movement of an inserted coupling module. Thecoupling modules can thus be attached to, and detached from, thecomplementary base module of the ultrasound probe using a rotational(screwing) action. The use of such a screw-fit fastening to provide themodular ultrasound probe is preferred because it allows consistent andsuitably good acoustic coupling by providing a consistent andhigh-tension clamping attachment between the delay line of eachindividual contact module and the wear plate of the base module. Suchscrew-fit attachment is also relatively low cost to implement.

In use, the storage tray 68 would be positioned at a known location andorientation on the CMM bed as shown in FIG. 3. The coupling moduleswould also be placed at known locations (i.e. in predefined slots 142)within the tray 68. In use, the CMM would manoeuvre the base module(e.g. the base module 58 of FIG. 3) to point downwards towards the tray68 and to be directly above the coupling module that is to be attachedto the base module. The base module would then be slowly moved downuntil it engages the coupling module, whereupon the base module isrotated (e.g. using rotational motion imparted by the rotary head 54 ofFIG. 3) so that the two mating threads engage and the coupling modulebegins to rise out of the tray as it screws on to the base module. Theexact point at which the coupling module becomes fully attached andtightly affixed to the base module can be determined by, for example,assessing the ultrasonic response generated during the manoeuvringprocess or by continuously monitoring the torque load placed upon therelevant axis of rotation of the rotary head 54. This torque load can bedirectly correlated to the electrical current demand placed upon therotational movement servo motor within the rotary head 54. Therotational angle at which each coupling module is interpreted as beingtightly attached to the base module can also be stored. The rotary head54 can return to this rotational angle to allow the coupling module tobe inserted back into a slot 142 in the storage tray 68 and thereafterdetached (unscrewed) by the opposite sense of rotational motion thanthat used for attachment.

It should be remembered that the screw-fit attachment method describedabove merely represents one possible method for allowing a couplingmodule to be attached to a base module. There are numerous alternativetype of connector that could be employed. For example, the connectioncould be provided by Luer joints, snap-fit mechanisms, embedded magneticfixtures etc. A magnetic clamping arrangement could comprise, forexample, an assembly of three strong and compact magnets equally spacedaround the circumference of the probe tip with polarity “++−” and amatching distribution of three magnets with polarity “−−+” around theperimeter of each coupling module stored in the dispensing tray. Thismagnetic attachment would provide a simple attachment for couplingmodule variants and provide only a single possible rotational angle ofattachment. Although the attachment of coupling modules to a base modulemay be implemented in an automated manner as described above, it shouldbe noted that it would also be possible for such attachment to performedmanually by an operator (e.g. by scheduling in a number of set breaks inthe inspection sequence),

Turning next to FIG. 5(c), a variety of different coupling modules180-191 are described. Each of the coupling modules is contained in anouter PTFE shell having the outer profile shown in FIG. 5(b) to allowstorage in the tray 68 of FIG. 5(a). The wear plate 194 of the basemodule that physically and acoustically couples to the delay line ofeach attached coupling module is also illustrated.

The coupling module 180 comprises an outer PTFE shell with a Rexolitedelay line and a hydrophilic-vinyl-elastomer tip. The coupling module181 comprises an outer PTFE shell with a Rexolite delay line and aspherical thermoplastic tip. The coupling module 182 comprises a taperedouter PTFE shell with a Rexolite delay line and a curved thermoplastictip. The coupling module 183 comprises an outer PTFE shell with aRexolite delay line and a thin latex rubber tip. The coupling module 184comprises an outer PTFE shell with a hydrophilic-vinyl-elastomer delayline and tip. The benefits of using a hydrophilic vinyl elastomer ballas a tip are explained in more detail below. The coupling module 185comprises a tapered outer PTFE shell with a Rexolite delay line and ahydrophilic-vinyl-elastomer tip. The coupling module 186 comprises anouter PTFE shell with a Rexolite delay line with an angled distal end towhich is attached a hydrophilic-vinyl-elastomer tip. The coupling module187 comprises an outer PTFE shell with a Rexolite delay line with anangled distal end to which is attached a thermoplastic tip. The couplingmodule 188 comprises a tapered outer PTFE shell with a Rexolite delayline with an angled distal end comprising a thermoplastic tip. Thecoupling module 189 comprises an outer PTFE shell with a thermoplastictip. The coupling module 190 comprises an outer PTFE shell with astepped distal end for holding a distorted hydrophilic-vinyl-elastomerball tip that also acts as a delay line. The coupling module 191comprises an outer PTFE shell, a Rexolite delay line and a thermoplasticmaterial that provides an object contacting tip.

FIG. 6 schematically illustrates how a preferred embodiment of theultrasound probe that comprises a single hydrophilic elastomer spherecan be implemented. As will be explained in more detail below, such anarrangement can advantageously be provided as part of a modularultrasound probe.

The ultrasound probe 200 shown in FIG. 6 comprises a single hydrophilicelastomer sphere 208. The hydrophilic elastomer sphere 208 may, forexample, be manufactured by synthesis and hydration of a cross-linkedhydrophilic vinyl elastomer, super absorbent polymer or hydrogel. Thehydrophilic elastomer sphere 208 is contained within an acousticallyabsorbent shell 212 (e.g. machined from PTFE). The sphere 208, when theprobe is loaded onto the surface, becomes uniformly deformed between thewear plate 214 and against the surface 216 of the object 218 beinginspected.

A transducer 210 comprising an active piezoelectric element generatesL-waves when driven by a train of high-voltage impulsive excitationpulses; e.g. negative going transition (NGT) pulses between 50-150V andof duration 1/2f. The characteristic acoustic impedance of the sphere208 (i.e. the “coupling element”) is such that there is a sufficienttransfer of acoustic energy into it from the transducer wear plate 214that acts as a matching layer. The relative impedances between thecontacting media (i.e. the sphere 208, the inspection part 218 and thesurrounding air) determine the proportional acoustic transmission (T)and reflection (R) at the interfaces between the probe and theinspection object in accordance with the equations:

$\begin{matrix}{R = \left( \frac{z_{2} - z_{1}}{z_{2} + z_{1}} \right)^{2}} & \left( {1a} \right) \\{T = {1 - R}} & \left( {1b} \right)\end{matrix}$

For most thin metallic parts, such acoustic impedance matching issuesactually have more influence on the resulting amplitude of the returnedreflection echo signals of interest than the inherent acousticattenuation exhibited within the coupling medium and the inspection partcombined. This is because the inherent acoustic absorption that willcause ultrasonic waves to attenuate whilst propagating through anymedium is frequency dependent and depends upon a number of factors suchas temperature of the medium and the inherent grain structure.

For the example shown in FIG. 6, depicting the interaction of ahydrophilic sphere 208 with an object 218 in the form steel plate, alarge proportion (e.g. >80%) of the initial acoustic energy is reflectedwithout ever entering the inspection part 218 due to the significantdifference in acoustic impedances between the hydrogel material of thesphere 208 (e.g. 1-3.5 MRayls) and the adjacent steel of the object 218(e.g. approximately 46 MRayls). However, for the proportion of energythat does propagate into the part 218, multiple reflections at the frontand back wall interfaces occur without significant acoustic leakage(i.e. only approximately 1.3% of the transmitted energy from the firstback wall reflection is returned to the transducer) and limited fall offin the signal amplitude levels of the repeated back wall reflections.

This example illustrates that selection of the coupling member (i.e.sphere 208 in this example) for mode-3 ultrasonic inspections involvesreaching a compromise condition in which just enough energy istransmitted into the part without the coupling being so efficient thattoo much reflected energy escapes from the part from the first back wallreflection because this would result in a low SNR for subsequent backwall echoes. It has also been found that the large proportion ofacoustic energy that never enters the part (i.e. the ultrasound energythat just rebounds within the hydrophilic sphere) can also be measuredand interpreted so as to infer something about the physical condition ofthe sphere within its surroundings or as it interacts with any othersolid, gelatinous or liquid bodies. As will be explained in more detailbelow, analysis of the delay line peaks of the acoustic spectrum thatare associated with reflections within the sphere can also be used toestablish contact between the sphere and an object. This can be usefullyexploited during an automated scan using coordinate positioningapparatus to obtain surface position information. This should becontrasted with the utilisation of A-scan signals from conventionalultrasonic delay line transducers, where such internal delay linereflections are more typically ignored or even time-gated outaltogether.

The hydrophilic sphere 208 arrangement illustrated in FIG. 6 has anumber of performance benefits, such as allowing both efficient andflexible point inspection measurements and also continuous scanningacross a complex geometry inspection surface using a platform with highaccuracy yet limited mechanical power. For example, the aqueous(hydrophilic) spheres 208 exhibit negligible acoustic attenuationsuggesting that they could be any size. Furthermore, their acousticimpedance value is well suited for ultrasonic transmission into metallicparts. In addition, the incompressible, deformable and almost gelatinousspheres are extremely soft and elastic so as to conform naturallyagainst any reasonably curved inspection surface. Also, the perfectlyspherical shape is ideal to achieve sufficient coupling yet retainpositional precision in point contact measurements against locallyplanar inspection surfaces.

The hydrophilic sphere arrangement can also be adapted to provide adegree of useful tapering or a concentrating effect towards thecontacting tip potentially altering the effective natural acoustic focallength of the probe. In other words, the elastic hydrophilic spheres canprovide a level of control over the beam divergence and direction of theprojected ultrasound into the part through accurate manipulation of theprobe loading and orientation on the inspection surface. Moreover, thespherical element provides the optimal structural shape to allow suchfragile solid material to undergo repeated elastic deformations causedby loading against the inspection surface while retaining mechanicalintegrity over short-term usage due to an even distribution of loadingstresses as the spheroid is compressed between the planar wear face andthe inspection surface. It is noted that the incompressible spherespreferably reside quite loosely within the shell and can elasticallydeform under load many times, each time returning to a perfect sphereafter removing the load. It is only when some tearing or splittingoccurs at the sphere surface that the element disintegrates.

The hydrophilic sphere 208 arrangement illustrated in FIG. 6 also hasthe important advantage that the water swelling chemical property of thehydrophilic polymer spheres allows them to release a controlled amountof water from the external surface. The amount of water released istypically of a quantity that will evaporate readily within theatmosphere. This subtle water release substantially improves acousticcoupling across all inspection surfaces since the water displacesunwanted air-pockets trapped between the probe and the surface roughmicro-structure. This is done without leaving any obvious residualliquids or contaminants and the need to apply gel couplants to the partis removed. Moreover, this water expulsion can be controlled throughknown organic synthesis methods and, as such, it also provides theadditional benefit that it is possible to continuously scan theultrasonic probe across an inspection surface without losing contact.More specifically, tangential forces introduced by any lateral motion ofthe scanning probe across the inspection surface (F) would in turninduce significant frictional resistive forces (N) in the contactingsphere that could potentially impair the positional precision of themoving probe or cause premature mechanical failure, if the couplingmember was totally dry. However, through the controlled water sweatingproperties of the hydrophilic spheres, sufficient liquid is releasedfrom the sphere to act as a natural and effective lubricant facilitatingsmoother continuous scanning motion across most surfaces in anydirection.

The hydrophilic sphere based arrangement described with reference toFIG. 6 can thus be seen to offer a number of benefits with respect toefficient point inspection and continuous automated scanningapplications. For certain geometries, e.g. involving non-parallel frontand back walls and/or where there is limited access, it may not alwaysbe practical or even physically possible to measure such parts byinducing L-wave beam refraction at the inspection surface at therequired refractive angle solely through re-orientation of the proberelative to the surface. In addition, irrespective of the natural focallengths and refractive beam angle produced by the aperture and probeorientation against the surface, the projected beam is inherentlydivergent and only focussed naturally as a result of the aperture sizeand operating frequency, without any explicit near-field refractivefocussing. Furthermore, although water release from these fixtures canbe controlled and is extremely modest, some applications (e.g.inspection of car assemblies) may require there to be no liquidresiduals including water. It is therefore advantageous to alternativelyor additionally provide coupling modules that comprise a rigid plasticrefractive lens or angle beam wedge material (e.g. acrylic orPolystyrene) bonded to a suitable soft coupling layer that forms theobject contacting tip. This “compound class” of coupling modules enablesselective yet rigidly fixed acoustic beam patterns to be generated bythe ultrasound probe.

Referring to FIG. 7, an example of a compound design of coupling moduleis shown. Again, the example shows the underlying principle of operationand could be implemented in the modular ultrasound inspection apparatusdescribed above. The ultrasound probe of FIG. 7 comprises apiezoelectric element 248 coupled to a normal incidence tapered delayline 250 that is bonded or loosely coupled to a thin soft coupling layer252 that protrudes and thus provides an object contacting tip. Thecoupling layer 252 in this example is a thin layer of latex rubber, forexample as used to make surgical gloves or similar items. Alternatively,the coupling layer 252 could be provided by a compressible oil-basedthermo-plastic. Both latex rubber and oil-based thermoplastics can beproduced that generate no residuals from deformation so would generateno liquid contamination during an inspection. When firmly loaded andacoustically coupled to the inspection surface, the normal incidencecompound ultrasound probe will generate a fixed known natural beamdivergence 256 within the part 254. However, a rigid refractive elementcan be specifically shaped asymmetrically in order to refract (i.e.steer) the L-wave at a set angle from the normal in order to accommodatemore complex internal geometries. This is in accordance with Snell's lawof refraction and methods may also be used to filter the slower shearwave modes.

FIG. 8 shows a further example of a compound design of coupling module.Again, the example shows the underlying principle of operation and couldbe implemented in the modular ultrasound inspection apparatus describedabove. The ultrasound probe of FIG. 8 comprises a piezoelectric element260 and a plastic planar-concave lens 262 that forms a rigid refractiveelement. The planar face 264 of the lens 262 is coupled to thetransducer wear plate 266 and the spherically concave face 268 iscoupled against (i.e. “cupped around”) a hydrophilic elastomeric sphere270. The refractive element (i.e. the planar-concave lens 262) mayalternatively be shaped as any required type of acoustic lens toconcentrate or focus the L-wave acoustic wavefront within the part at apoint in the near-field. The ultrasound probe of FIG. 8 allows, giventhe relative sound speeds within each medium, the L-wave to be focussedwithin the part 272. For example, the L-wave may be focussed at point Pon the back wall of the part 272. This arrangement provides a similarA-scan response to that obtained using a spherically focussed probe ofan immersion system; the hydrophilic elastomeric sphere 270 replaces thewater in which the probe would be submerged.

FIGS. 9(a), 9(b), 9(c), 10(a) and 10(b) are various design images andphotographs of components of a modular ultrasound probe of the typedescribed above that comprise a coupling module having a couplingelement in the form of a hydrophilic elastomeric sphere.

As explained above, the modular ultrasound inspection apparatusdescribed above comprises a plurality of coupling modules that can beattached to a common base module. FIG. 9(a) shows a design image of thedistal end of the base module 290. The cylindrical body of the basemodule includes on its outer surface a threaded connector portion 292.As will be explained below, the threaded connector portion 292 allowssuitably arranged complementary coupling modules (e.g. as shown in FIGS.9(b) and 9(c)) to be screwed onto the base module.

Turning to the coupling modules, the coupling element (e.g. anhydrophilic elastomeric sphere) that touches the part to be inspectedand any required delay line (e.g. a normal delay medium or a refractivedelay medium) is preferably retained within an acoustically absorbentshell. The provision of such a strongly absorptive shell means thatprojected L-waves being used for the thickness measurements can dominateover other waves (e.g. reflected waves from the sides of the couplingelement). This can, for example, result in more compact refractive wedgedesigns being possible. Examples of absorptive shells for housing ahydrophilic elastomeric sphere will now be described with reference toFIGS. 9(b) and 9(c), but it should be noted that similar absorptiveshells could also be used for different types of coupling elements.

FIG. 9(b) shows a base module 293 made to the design illustrated in FIG.9(a) and attached to a coupling module 295 comprising an acousticallyabsorbent shell 294. The shell 294 suppresses internal acousticreflections and retains a hydrophilic elastomeric sphere 296. In thisexample, the shell 294 is precision-machined in glass-filled PTFE (e.g.PTFE sold under the Teflon® brand). Alternatively, pure PTFE or someother suitable anechoic polymer could be used. It should also be notedthat a range of acoustic polymers specifically designed for anechoicabsorption of high frequency acoustic reflections are commerciallyavailable. For example, Aptflex F28 from Precision Acoustics is a highfrequency anechoic acoustic absorber material used for test tank liningswithin immersion systems and would thus be a suitable material for theabsorbent shell as it offers extremely favourable acoustic attenuationattributes for any unwanted internal ultrasonic echoes. However, PTFEhas the advantage of being a low friction material that is ideal forallowing the hydrophilic spheres to move freely within the shell as itcompresses under loading on the surface, without any tendency to stickto the inner surface of the restraining shell. An internal screw thread(not visible in FIG. 9(b)) forms a second connector portion that allowsattachment of the coupling module 295.

FIG. 9(c) illustrates a compound coupling module 300 formed from a shell302 that encases a delay line 304 and retains a hydrophilic elastomericsphere 306. The shell 302 is, like the shell 294 illustrated in FIG.9(b), formed from a precision-machined in glass-filled PTFE. An internalscrew thread 308 forms a second connector portion that allows attachmentof the coupling module 300 to the first connector portion comprising thecomplimentary screw thread 292 formed on the base module 290.

As shown in FIGS. 9(a) and 9(b), the PTFE shells 294 and 302 encase themajority of each coupling module with only a fraction of the hydrophilicelastomeric spheres 296 and 306 that provide the coupling elementprotruding from the end of the shells in order to make direct contactwith the inspection surface. In the case of the compound coupling module300 of FIG. 9(c), the detailed shape of the PTFE shell 302 in thevicinity of and around the circumference of the protruding softhyper-elastic coupling material of the delay line 304 has an effect uponthe coupling performance achieved due to the effective compressiverestraint of the soft coupling element 306 as it is trapped within thestructure. It also has an effect on the likely service-life of theconsumable item due to an increased probability of tearing thehydrophilic elastomeric sphere 306 in cases where this restraint causesa concentrated stress profile across the soft coupling material. It isalso noted that the PTFE shell provides some useful general protectionto soft perishable coupling materials.

The use of glass-filled PTFE to form the shells 294 and 302 of FIGS.9(b) and 9(c) respectively also aids attachment of each coupling moduleto the associated base module. In particular, the PTFE allows smooth,automated screw-fastening between each coupling module and the basemodule. The mechanical screw-fit assembly can thus be designed usingmaterials (e.g. a PTFE shell for the coupling module and steel for thebase module) that promote smooth interaction between the mating parts.Moreover, the coupling module dimensions can be set so that when thescrew-fit assembly becomes tight the planar wear face or wear platewithin the base module makes consistent contact with an adequateclamping tension force with the inner coupling material of the couplingmodule. The use of such a PTFE shell allows the formation of delay lineswithout the need to micro-machine grooves to suppress reflections,allowing cheaper high volume injection moulding or vacuum castingmanufacturing methods to be adopted.

Referring to FIGS. 10(a) and 10(b) photographs of one embodiment of theultrasound probe 330 are provided. The ultrasound probe 330 shown inFIGS. 10(a) and 10(b) is configured to be attached to a rotary head thatis in turn mounted on the moveable quill of a CMM. In particular, themodular ultrasound probe shown in FIGS. 10(a) and 10(b) is arranged forattachment to a two-axis rotary head (e.g. a REVO® head of the typedescribed above with reference to FIG. 3). It would, of course, bepossible to attach such an ultrasound probe to other measurementsystems.

The ultrasound probe 330 comprises a base module that includes a mainbody portion 321 that contains all the transmit-receive (Tx-Rx)electronics required to drive the piezo-electric ultrasound transducerand digitally record the acoustic response to such excitations. The mainbody portion 321, which is provided at the proximal end of the probe 330that attaches to the CMM comprises optional electromagnetic shielding toprotect the transmit-receive electronics. The main body 321 can alsocontain all the electronics required to power the probe and communicatecontrol data and activation commands to the probe (e.g. to scheduleultrasonic measurements). Power and/or control data, includingultrasonic data and thickness measurement results, may be passed throughthe rotary head communication channels.

The main body 321 also comprise a thin, elongate and rigid carbon-fibretube 323 that extends along the probe's axial length. The distal end ofthe tube 323 carries the ultrasound transducer and a first connectorportion 322 for attachment to a coupling module. FIG. 10(a) shows thebase module without a coupling module attached, whilst the enlarged FIG.10(b) shows a coupling module 332 attached (i.e. screwed onto) the firstconnector portion 322. A high-frequency and shielded coaxial cable (notshown) extends internally along the carbon-fibre tube 323 so as toelectrically connect the Tx-Rx electronics in the main body 321 to thetransducer provided near the tip. This transmits the high voltage pulsesfrom the Tx-pulser electronics to the transducer in order to generateacoustic waveforms and also carries the measured analogue voltagesignals from the transducer back to the Rx-electronics to be digitisedand recorded. Although the physical form of the probe is advantageouslychosen so that the electronics module is compact and contained withinthe main body close to the CMM's measurement head, the overall lengthcan be specifically selected by varying the length of the carbon-fibretube and/or the crank angle so that the transducer module and probe tipcan access hard-to-reach part geometries.

Referring to FIG. 11, the transmit-receive (Tx-Rx) electronics containedwithin the main body 321 of the ultrasound probe 330 described withreference to FIGS. 10(a) and 10(b) are described.

FIG. 11 depicts one embodiment of the analogue and digital electronicmodules that can be provided within the ultrasonic probe. An analogue“pulser” circuit 350 is provided that is capable of generating repeatedtrains of high voltage (50-150V) a.c. analogue signals (e.g. NGTpulses). Although the pulser 350 is provided, a more sophisticateddigital waveform synthesizer could alternatively be employed to generatefrequency or amplitude modulated waveforms to drive the piezo in moreattenuating environments. The high voltage pulses generated by thepulser 350 effectively drive the piezoelectric active element 356 withinthe transducer of the probe to output the required ultrasonic waveforms358, but without exceeding the maximum voltage for such a thin fragilepiezoelectric element. Each pulse activation may be instigated andprecisely controlled in time by an enable signal sent to the “pulser”circuitry 350 from an FPGA 352 or equivalent processor. For everyactivation, a fast T/R switch 354 allows the device to instantaneouslyswitch between the transmit mode and the longer duration receive modeduring which time the system acquires and digitally records the acousticresponse to the transmitted pulse measured by the reciprocalpiezoelectric element 356.

The amplitude level of the received signals of interest can varysignificantly, so a variable gain amplifier (VGA) 360 is optionallyprovided to induce SNR gain across the acquired A-scan response in orderto amplify the signal prior to digital acquisition. Moreover, toequalise the variability within each A-scan response due to propagationloss or attenuation with some materials, a form of automatic gaincontrol (AGC) known as distance-amplitude correction (DAC) may also beimplemented. The amplified A-scan is then digitized using a suitablywide dynamic range (e.g. 12 bit) analogue-to-digital converter (ADC) 362where sufficient over-sampling above the Nyquist rate is provided as thesample rate fundamentally effects the temporal resolution of themeasurement system and thus the accuracy of the thickness measurement;e.g. a sampling rate of 125 MHz or higher may be suitable for a 20 MHztransducer. The encoded digital waveforms from the ADC 362 may alsorequire band pass filtering using a digital filter, for example a loworder FIR with a pass band matching the operating frequency of thetransducer. The Tx-Rx electronics are designed so as to minimise allpossible sources of electronic noise that may be observed within eachindividual A-scan. Such uncorrelated noise is most effectivelysuppressed by averaging across N successive repeated A-scan measurements(ie. providing a theoretical IN SNR gain).

Referring to FIG. 12, an example will now be described of the input datarequirements for inspection planning software that is employed tocompile a sequence of automated movement instructions for the CMM andultrasound inspection apparatus described with reference to FIG. 3.

It is known in surface contact metrology using a surface contact(scanning or touch trigger) probe to employ software that utilisesnominal CAD data models of the inspection part to automatically generatea part program that plans and executes the measurement moves. Forexample, high-resolution continuous sweep scan measurements of a turbineblade can be performed using the ApexBlade software sold by Renishaw plcto generate a part program in the industry standard DMIS language forcontrolling the CMM. Similar CNC software, for example using andgenerating accepted high-level CMM control software languages (e.g.DMIS), may also be used to automatically plan and schedule theultrasonic probe inspection. Any such inspection plan, whether performedautomatically or manually, preferably includes some form of detailedpart-specific inspection planning or scheduling.

A first requirement is to define where the ultrasound measurements arerequired. This may be achieved by defining, prior to ultrasonicinspection, an inspection plan that defines the position of allmeasurement nodes, linear sections (B-scan lines) or defined inspectionareas across the target inspection part upon which ultrasonic inspectionmeasurements are to be taken. This process may use external formmeasurements of the inspection part performed using a surface contactmeasurement probe of known type. After defining the measurement nodesand with knowledge of the detailed inspection part geometry and theavailable mechanical degrees of freedom offered by the automationplatform carrying the ultrasound probe, the type of ultrasound proberequired for the measurements can be determined. For example, it mightbe possible to use only a normal axis ultrasound probe (e.g. asdescribed with reference to FIGS. 4(a) and 4(b)) or a crank-angledultrasound probe (e.g. as described with reference to FIGS. 4(c) and4(d)) may be necessary for some or all measurements. If more than oneultrasound probe is required, an automatic probe changing routine may berequired.

As explained above, the modular ultrasound probe includes anexchangeable coupling module. As shown in FIGS. 3 and 5(a), the couplingmodules may be stored in a storage tray and hence be automaticallyattached to, and detached from, the base module of the ultrasound probeas required. The inspection plan may thus include selecting the couplingmodule or modules that are most beneficially employed for measurementsacross different geometries within the inspection part. Each couplingmodule may also have a limited lifetime (e.g. it may be a consumable orlimited lifetime item) and the planning process may thus include areplacement strategy for refreshing such coupling modules. For example,a purely scheduled changing strategy would likely involve deciding uponthe optimal coupling module for each section of the part in terms ofcoverage and scan performance and scheduling a set number ofreplacements within the inspection to eliminate the possibility of usinga damaged coupling module. A predictive replacement strategy wouldinvolve replacement of a coupling module only when damage or sub-optimalperformance is detected; this would preferably involve determining theoptimal designs for a specific geometry and ensuring that enough of eachdesign is available to cover the likely number required. A mixture ofscheduled and predictive replacement could also be employed.

After determining the measurement nodes, ultrasonic probe changes andcoupling module changes for the planned inspection, the optimal movementpaths can be generated. This process preferably ensures that the probemovements are suitably blended together so that the probe does notcollide with any obstacles (e.g. the part, fixturing or granite bed).For a predictive coupling module changing strategy, it is important thatwherever the probe is located within the measurement volume when damageis detected to the current coupling module, a safe movement pathsequence can be called in order to return the probe to the storage tray.A list of the internal wall thickness measurement nodes on theinspection surface can then be compiled and the ultrasound path defined.

After attaching the base module of the ultrasonic probe to the moveablemember (e.g. two-axis rotary head) of the CMM, tests may be performed toensure that the piezo probe is functioning correctly. The axialalignment and position of the base module when attached to the rotaryhead can be assumed to be consistently fixed with sufficient accuracy toautomatically attach and detach coupling modules from a storage tray.This is because the base module of the modular ultrasonic probe is asubstantially rigid body and can be attached to the measurement headusing established kinematic joints. However, calibration of the positionof the coupling element (i.e. tip) of the coupling module is preferablyperformed after attachment of a coupling module to the base module. Thisis in order to accurately determine the position of the sensing tip(i.e. the coupling element) within the coordinate system of the CMM.

FIGS. 13(a) and 13(b) show an example universal calibration artefactthat can be used for sound speed calibration, XY positional calibrationand other calibration tasks. It should be remembered that this is merelyone example of a suitable calibration artefact and other calibrationartefacts and techniques may be used instead.

FIG. 13(a) shows a two-dimensional cross-sectional representation of thecalibration block 400 that is also shown in three-dimensions in FIG.13(b). The calibration block 400 is a precision machined artefact thatincorporates planar orthogonal faces 402 so that it can be placed on thebed of a CMM and its position accurately measured (i.e. datumed) interms of XYZ position and orientation using a surface measurement(scanning or touch trigger) probe. The block 400 also has a flat planartop surface 404 with a central dimple feature 406 that can be located inthe CMM volume using a surface contact (e.g. a touch trigger orscanning) probe. The calibration block 400 is hollow and contains aninternal conical surface 408 with a shallow slant angle (e.g. 5-10degrees) relative to the top surface plane 404. The apex of the conedefined by the conical surface 408 is concentric to the XY coordinate ofthe central dimple feature 406.

In use, the position and orientation of the calibration block 400 in thecoordinate system of the CMM is determined by a conventional metrologydatuming process. For example, a datum point and the principle axes canbe determined from a orthogonally planar section of the block by takingat least 6 touch points (e.g. three points defining the Z-plane, twopoints defining an x-line and one point defining a Y-point) using aconventional touch trigger probe. Once the position of the calibrationblock has been found in this manner, the position of the ultrasoundprobe tip within the CMM volume can be determined.

In particular, the position of the block 400 within the CMM volume canalso be determined from two sets of measurements taken using an acousticprobe comprising a coupling element in the form of a hydrophilicelastomeric sphere. In the first measurement, the position of the tip ofthe acoustic probe (and hence the position of a point on the surface ofthe block) is determined in the z-axis by moving the acoustic probe topoint downwards to a point above the top plane of the calibration block400. In other words, the probe is moved in the [0 0 −1] direction bynulling the head to face the top face normal vector [0 0 1]. Theacoustic probe is then loaded down onto the top planar surface 404 ofthe block 400, which resides at a known Z-height, by slowly moving theCMM quill down in the Z-direction. By repeated loading of the probe onto and off the top surface 404 it is possible to estimate theZ-coordinate at which the probe makes tangential contact with thissurface; this is achieved by analysis of the acoustic signal generatedfrom reflections from the hydrophilic elastomeric sphere as described inmore detail below. This first measurement thus enables an accuratedetermination of the Z-position of the tip within the CMM volume.

Secondly, to estimate the XY location of the tip of the acoustic probewithin the CMM coordinate system, a sequence of ultrasound thicknessmeasurements (e.g. at least 6 for a unique solution) are made across thetop surface 404 of the calibration block 400 above the internal conicalfeature. Again, the probe is arranged to point downwards (i.e. bynulling the probe head) and the XY position of the probe is recorded ateach measurement node. The block thickness is then calculated at eachmeasurement point and the set of thickness measurements acquired in 3Dare mathematically fitted to a conical shape, for example using theLevenberg-Marquardt (LM) algorithm or any linear or non-linearleast-squares conical fitting algorithm. This fitting process revealsthe offset between the XY estimation of the apex of the fitted cone andthe actual XY location of the dimple.

In a preferred embodiment, the calibration block 400 shown in FIGS.13(a) and 13(b) may be machined using the same type and grade ofmetallic material found in the part to be inspected. The block 400 canthen also be used to perform sound speed calibration in order toestimate the wall thickness of any subsequent part measurements. Thiscan be achieved by measuring the time delay for the known thicknesssections around the perimeter of the calibration block. It is noted thatsound speed may alternatively be measured directly from known solidsections of the part to be inspected (e.g. on the root of a blade ornear the aerofoil) so as to minimise variability sources. This isbecause the sound speed calibration likely provides the largest sourceof measurement error within the thickness measurement calculation; e.g.due to temperature differences within the environment, crystallographicorientation or differences in density/porosity microstructure.

Mode-3 gauging is a preferred method for thickness calculation using themodular acoustic probe described above as it is substantially unaffectedby variability in the coupling element (e.g. variations in thepropagation path through the hydrophilic elastomeric sphere). However,the calibration techniques described above can also be used in mode-1 ormode-2 gauging. In such gauging techniques, absolute propagation timedelays for the first back wall reflection across a range of thicknessescan be used to directly obtain any subsequent thickness measurementswithin the measured range by linear interpolation. This may have alimited number of uses, for example measuring much thicker and morehighly attenuating parts with a coupling module having a rigid constantdelay line and a very thin coupling element (e.g. latex rubber) so as toreduce variability.

The calibration block 400, which may represent accurately the part beinginspected, can have further uses within the context of highly automatedpoint measurement and continuous scanning. As described below, theseadditional uses include probe contact detection, normal probe loadingestimation, thickness measurement and wiggle movement adaptation.

For probe contact detection, the calibration block 400 provides aconvenient known geometry and identical planar surface target from whichthe probe with any coupling module attached can measure and calibraterelevant variations in the A-scan waveform for the probe, at the pointwhen the tip comes into contact with the surface. As will be describedin more detail below, this technique exploits changes in amplitude,phase, fine-scale shape, frequency and/or time of arrival (TOA) of theinternal reflection echoes from within the coupling module which areextracted from a continuous repeated train of measured A-scans so as toinfer probe tip contacts with any solid body. For example, it has beenfound that the amplitudes of second and third delay line reflections(DL2 and DL3) within a rigid delay line are extremely sensitive to anyprobe tip contacts from a solid body.

For normal probe loading estimation, the use of internal reflectionsfrom the coupling modules extracted from a repeated train of A-scans toinfer such probe tip contacts can be extended to measure and calibratethe normal loading of the probe on to a planar surface, using such acalibration block. By loading the probe slowly on to the calibrationblock whilst continuously recording A-scans at a high rate, waveformfeatures from the coupling module's internal reflection echoes as thesoft coupling element deforms against the inspection surface can beextracted and stored with the positional information of the tip andsurface. These features adequately define how the physical condition ofthe soft coupling element within the coupling module changes undernormal loading conditions and can therefore infer the loading conditionwhen making subsequent measurements across the inspection surface. Suchnormal loading calibration to infer displacement or deflection withinthe soft coupling element of the coupling module can be very useful forensuring precise manipulation of the probe against the inspectionsurface (e.g. to alter the effective aperture or angle of incidence ofthe probe) or for continuous scanning of the manoeuvring probe acrosscomplex and/or unknown geometry surfaces. Substantially constant loadingconditions across an unknown topography can thus be attained using suchinferred measurement at a high rate as direct feedback for positionaladaption in the CMM and/or active head controller.

For thickness measurement, accurate and computationally efficient timedelay estimations between back wall reflection echoes are possible. In apreferred embodiment, this time delay estimation process can involveimplementation of a form of generalised cross-correlation (GCC)algorithm that convolves stored or extracted replicas of back wallechoes across the measured A-scan in order to accurately sharpen thetime delay estimation between successive back wall reflections. Thisspectral technique exploits the overall shape of the back wallreflection waveforms including amplitude and most notably phase (e.g.using phase transform pre-whitening) to determine their time of arrivaland thus the accurate time difference between successive echoes. Assuch, the calibration block 400 can be used to measure and store anextended set of representative replica waveforms of back wall echoesthat can be used during the inspection.

It should also be noted that the same methods for measuring the timedelay between successive back wall reflections in Mode-3 gauging can beused for precision measurement of the internal reflection echoes fromthe deformable coupling modules. More specifically, it is beneficial tostore template signatures of the internal reflection peaks duringcalibration that can be used during the subsequent inspection to extractthe accurate time of arrival of the internal reflection peak.

For wiggle-movement adaption, it is identified that ultrasonic couplingto any surface is not entirely deterministic in that the best SNR is notalways achieved for a normal incidence L-wave transducer simply byloading the device normally to the surface with the maximum forceavailable. Stochastic processes can also influence the SNR achieved fora probe being loaded on to an inspection surface. For example, theprevailing micro-structure, humidity and temperature conditions caninfluence how air becomes trapped between the probe and the inspectionsurface, introducing significant potential variability in the ultrasonictransmission. For these reasons, it is known in both manual andautomated ultrasonic NDT measurements (e.g. using the Marietta-NDT 5-550system) to apply some fine-scale adaption to the probe orientation (e.g.rolling and/or twisting), whilst keeping the probe tip stationary on thesurface, in order to optimise the received signal level. Moreover, withan accurate and infinitely indexing automation platform (e.g. a CMM witha 5-axis active head), it becomes possible to determine favourablesequences of such fine-scale probe movements (e.g. rolls and twists) fora specific inspection condition. It is therefore further highlightedthat the calibration block 400 provides such a known and representativesurface in which the probe can determine or algorithmically learn (e.g.using optimisation, clustering or artificial neural classifiers) theoptimal fine-scale sequence of wiggle movements for a modular acousticprobe that can then be adopted during the subsequent part inspection.

FIG. 14 illustrates a process for measuring an aerospace fan bladedisc/hub 450 using five-axis CMM apparatus of the type described withreference to FIG. 3 and the crank-angle variant of modular acousticprobe 109 illustrated in FIGS. 4(c) and 4(d).

After the modular acoustic probe has been mounted to the two-axis rotaryhead of the CMM and the necessary calibration procedures completed, theprobe 109 can be used to take measurements across the part. The probe109 can take both point measurements 441 and continuous scanningmeasurements 442 during the inspection of the blade 450, as necessary.For example, measurements may be taken at a set of spatially distinctnodes (e.g. at twenty locations distributed across each blade) and/orcontinuous scanning measurements may be acquired (e.g. by moving theprobe along a path on the blade's surface whilst collecting measurementsat a 1 mm pitch).

As shown in FIG. 14, the crank angle allows the ultrasound transducer(and hence the projected L-waves) to be orientated at a fixed angle awayfrom the longitudinal axis of the probe 109. Despite having closelyspaced adjacent blades, this cranked probe arrangement allows theultrasound energy to be directed normally to the surface of the blade450 using the five degrees of motion (three translational axes X, Y, Zand two rotary axes A,B) provided by the CMM and rotary head. Somegeometries and scanning regimes may, however, benefit from providing afurther rotational axis (C) around the primary probe axis for seamlesscontinuous scanning.

Referring to FIG. 15, the A-scan waveforms generated during thicknessmeasurements of a planar part taken using a modular ultrasound probehaving a coupling module comprising a hydrophilic elastomer sphere areillustrated.

FIG. 15(a) depicts a hydrophilic elastomer sphere tip 462 of a modularultrasonic probe 464 being moved normally towards a planar inspectionsurface 466 by the CMM on which it is carried. The A-scan illustrated inthe graph of FIG. 15(a) shows the amplitude of the received (returned)ultrasound pulse echoes as a function of time prior to the probe makingcontact with the surface.

The first peak 470A corresponds to the excitation pulse generated by thetransducer of the probe. The later peaks 470B, 470C are the time delayedinternal reflection peaks from within the uncompressed hydrophilicsphere. These consistent A-scan waveforms of FIG. 15(a) thus show a“rest-state” condition from within the undeflected coupling module whenit surrounded only by air. That is, the probe tip (i.e. the hydrophilicelastomer sphere 462) is yet to contact the inspection surface so thereare no external mechanical forces acting on the sphere. A train of suchA-scans can be performed at a high repetition rate (e.g. 1000-2000 Hz).As with the A-scan for a delay line transducer illustrated in FIG. 2, itis noted that the time window defined between the first and second peaks470A and 470B provides the primary measurement window for the probe. Therepetition rate should, however, not be so high as to cause significantinterference between successive pulses.

FIG. 15(b) depicts the exact point at which hydrophilic sphere tip 462of the probe 464 first comes into contact with the planar inspectionsurface 466. Even though there will be no noticeable shape distortion ofthe sphere at the instant of very first contact, a clear and immediatechange in the measured A-scan waveform occurs. Firstly, the reflectionpeaks 480B and 480C (i.e. the time delayed internal reflection peaksfrom within the uncompressed hydrophilic sphere) show a reduction inpeak amplitudes. This is more pronounced for the second reflection peak480C. Secondly, as the probe begins to make a more significant contact,the peaks begin to shift marginally to the left (i.e. towards the t=0excitation pulse 480A). Thirdly, even with light contact between thehydrophilic sphere 462 and the hard inspection surface 466, a pluralityof measurable reflection peaks 482A, 482B and 482C from successive backwall reflections of the part can be observed within the primarymeasurement window of the A-scan.

It should be noted that only three reflection peaks 482A, 482B and 482Care shown in FIG. 15(b) for clarity (i.e. there are likely to be morethan three such reflection peaks) and that these back wall reflectionsarise due to the coupling properties of hydrophilic spheres. Inparticular, an important benefit of the hydrophilic sphere basedultrasound probe is that it can provide sufficient delay for thin partmeasurement whilst only requiring modest probe contact with theinspection surface. This is a direct consequence of the ability to fillthe air gap between the probe and the surface on account of the softconformal contact properties of the hydrophilic sphere and theirpartially wet to the touch feel.

The change in the A-scan that results from contact with a surface thusallows the ultrasound probe to make surface contact measurements. Thiswill be explained below in more detail. Although the back wallreflections 482A-482C may be adequate to provide a Mode-3 thicknessestimation, it is preferred to load the ultrasound probe further ontothe inspection surface in order to establish increased acoustic couplingwith the part. In particular, further loading allows optimal couplingcontact (also referred to herein as the coupling ‘sweet spot’) to beobtained; this optimal coupling is revealed by the combination ofreduced reflection peaks from within the coupling module and increasedback wall reflection peaks.

FIG. 15(c) shows the consequence of such further loading of thehydrophilic sphere 462 of the modular ultrasound probe 464 on to theinspection surface 466. As can be seen from the A-scan plot, thereduction in the amplitude of the coupling module reflection peaks 490Band 490C (i.e. the time delayed internal reflection peaks from withinthe uncompressed hydrophilic sphere) is more pronounced. This isaccompanied by a noticeable increase in the SNR of the back wallreflections of primary interest to the thickness measurement (i.e. peaks492A, 492B and 492C). It can also be seen that both the internalcoupling module reflections (i.e. peaks 490B and 490C) and the back wallreflections (i.e. peaks 492A, 492B and 492C) within the measurementwindow are further shifted in time towards the t=0 transmit pulse 490Aas the probe is loaded further on to the surface and the sphere becomesprogressively more deformed. However, the delay between the successiveback wall reflections (i.e. peaks 492A, 492B and 492C) is unchanged.

FIG. 15(d) shows further loading of the probe onto the surface past the“coupling sweet spot” mentioned above. There is often observed a furtherreduction in amplitude of the coupling module reflection peaks 500B and500C (i.e. the time delayed internal reflection peaks from within theuncompressed hydrophilic sphere) but no substantial change in the backwall reflection signal (i.e. peaks 502A, 502B and 502C). The peaks ofinterest can also be seen to be further shifted towards the initialTransmit pulse at T=0 (i.e. main excitation peak 500A). Further loadingof the probe onto the surface, beyond the “coupling sweet spot”, thusgives no further improvement in the SNR of the back wall reflectionsignals. In addition, such further loading means that the spheredeformation can approach a condition in which temporal overlap betweenthe transmit and receive waveforms of interest is observed within theA-scan or the hydrophilic sphere is damaged.

The A-scan data illustrated in FIGS. 15(a) to 15(d) can be subjected toa variety of signal or data processing methods to allow automaticdetection of variations in a continuous train of A-scans whilst theprobe is being manoeuvred on to an inspection surface. Waveforminformation extracted from these A-scans, and in particular thetransient waveforms from internal reflection echoes from the hydrophilicsphere, provides a sensitive and robust method for detecting exactlywhen the tip of the ultrasound probe makes contact with any other body.This surface contact information has several uses.

A first detection method involves capturing a single reference A-scanwith the probe positioned at some ‘null’ position within the CMM volumewhen it is known that the tip of the ultrasound probe is not in contactwith a solid body. This reference waveform (such as the waveformillustrated in FIG. 15(a)) only contains internal reflection peaks fromthe uncompressed sphere and represents a condition defined by no tipcontact. Importantly, due primarily to the very high elasticity of thesoft coupling sphere, it is observed that this A-scan waveform shape isconsistently returned after any tip contact loading from any solid bodyhas been removed. The A-scan segments containing the coupling modulereflection peaks (i.e. the internal reflections from the hydrophilicsphere) can be extracted and repeatedly compared or differenced againstthe same time-gated segments from a continuous train of A-scans measuredas the probe is manoeuvred prior to surface contact. Monitoring fordifferences in this manner can be used to automatically detect when tipcontact with an object occurs.

Automated detection decisions can be indicated based upon any suitabledetection criterion. Although some scenarios for the probe could dictatea more sophisticated or adaptive detector (e.g. CFAR, Bayesiandetectors), a simple square-law energy detector with an absolutepre-determined hard detection threshold can suffice in many scenarios.This approach is valid because the continuous train of measured A-scanwaveforms exhibit quite negligible measurement-to-measurementvariability whilst the probe is being manoeuvred in free space at anyspeed or through any complex movement that the CMM and/or head caninduce. Moreover, any contact between the probe tip and a solid bodyinduces instantaneous and quite large changes across the observedinternal reflections echoes from the hydrophilic sphere. The automatedcontact detection algorithm within the probe may also analyse any numberof reflection echoes beyond the first echo return. For example, the2^(nd) and 3^(rd) reflection waveforms can often change more noticeablyin amplitude than the initial first echo return on such contacts (e.g.as seen in FIG. 15(b)) and thus these waveforms can also provide asensitive indication of any touch contact event. The pulse generationrepeat rate is preferably selected so that interference from previouspulses is minimised.

The signal features that define the coupling module internal reflectionechoes which are extracted from each A-scan and used as the input datawithin the detector may simply be related to differences in accumulatedwaveform energy. However, it is noted that the signal metrics employedmay vary in their ability to affect robust yet sensitive real-time tipcontact detection. Other waveform metrics including peak voltage, signalkurtosis (i.e. fourth statistical moment), RMS, FFT and AR coefficientscould equally-well be used, although any signal features can beextracted for use within the detector. Any such algorithms for detectingmeaningful variations in the A-scan waveform in order to infer tipcontact events will, in practice, require minimal computation as thecomparison or detection decision based upon a differencing process needonly be computed across the short time-gated segmented windows extractedfrom within each A-scan containing the coupling module internal echopeaks. Thus, in practice, the rate at which the probe can report tipcontact status is more fundamentally limited by the frequency at whichthe A-scans can be generated, rather than detector computation. Itshould be noted that the A-scan generation rate depends upon thethickness and L-wave sound speed of the medium within the couplingmodule and the resulting transit time needed to record at least thefirst two reflections from the hydrophilic elastomer tip. Due to therelative simplicity and low computation in the detection task, thefrequency at which contact status information can be reported by theprobe and sent to peripherals (e.g. the CMM or measurement headcontroller) can be relatively high (e.g. up to 2000 Hz). However it isnoted that increasing the repeat rate so that a new transmit pulse isinduced before the previous reflections have more significantlyattenuated can cause a set of additional unscheduled/spurious noisepeaks of diminishing amplitude to proliferate into successive A-scans.These can be more significant in amplitude between the transmitted pulseand the first internal reflection, although they can be effectivelyfiltered out within the primary measurement window using the primarysignal processing method for extracting the required time delay betweensuccessive back wall reflections.

Generating tip contact status data at such a high rate allows theautomated inspection system to respond or mechanically react relativelyquickly to any unscheduled tip contact events that the probe mayencounter unexpectedly during any type of movement induced by theautomation platform (e.g. motion of the CMM and/or measurement head).For example, if a tip contact were to be detected whilst the probe wastravelling along a linear trajectory at a typical scanning speed (e.g.100 mm/sec), the minimum possible travel into the obstruction, assumingno latency in sending the interrupt command to the CMM and head to stopthe movement, would equate to a deflection in the soft coupling elementof the coupling module of only about fifty microns. Even allowing forsome reasonable time latency in affecting the command for the CMM and/ormeasurement head to halt such a movement, the likely amount ofdeflection within soft coupling element of the coupling module would beorders of magnitude within the nominal maximum allowable deformationbefore which any damage to the hydrophilic sphere tip or rigid probewould be induced. The combination of high temporal resolution contactstatus data and the positional tolerance provided by the soft-elastictip thus dictates a low probability of the occurrence of significantundetected impact damage to the probe tip.

As explained above, the sensitive touch contact capability is veryuseful for navigation of the probe within CMM space. This is especiallysince the probe is a rigid body and has no other sensing modality socould be easily damaged. However, the ability of the ultrasonic probe togenerate useful surface interaction data at a very high rate goes beyondsimple binary contact detection. As will be described below, signal anddata processing methods have been devised that allow the probe to beused within any inspection as a simple yet sensitive touch probe capableof generating Cartesian point-cloud measurements describing the externalform of an inspection part. This basic touch point capability has directbenefits to the metrology inspection conducted by the ultrasound probe(e.g. time saving) as well as wider applications (e.g. sensitive andaccurate measurement of soft gelatinous parts with difficult opticalproperties that could not be measured easily with a conventional touchprobe or an optical scanning probe). In addition, the loading conditionof the probe against the inspection surface can be continuouslyestimated by direct exploitation of the internal coupling module delayechoes within the measured A-scan. This has direct and importantbenefits for both more controllable point measurements and continuousmove scanning inspections undertaken using the ultrasound probe.

It should be noted that the touch capability could be further refined byexciting the probe's piezoelectric active element with a continuoussinusoid signal during manoeuvres. For example, the hydrophilic spheretip could be driven with a continuous sinusoidal excitation at theresonant frequency, say 20 MHz. Any dampening detected in the resonancewhen the sphere is contacted by any solid could be detected.

Processing methods will now be described with reference to FIGS. 16 to19 that allow the modular ultrasound probe to be used as a basic touchtrigger probe that can take useful point cloud measurements across theexternal form of an inspection part. For example, this would allow theprobe to adequately survey the orientation of a surface for a subsequentthickness measurement at a point by taking three touch measurements inclose proximity around the required measurement node so as to estimatethe surface normal. As described above, touch contacts may be detectedwith the probe by continuous monitoring of the internal reflections fromthe hydrophilic sphere of the coupling module so that any meaningfulvariation in these echo waveforms (e.g. Peak amplitude, Phase,time-of-arrival changes) from the calibrated non-contact referencecondition can be detected.

FIG. 16 illustrates a modular ultrasound probe 546 moving at a constantvelocity towards a solid block 547 with the probe tip 548 kept at aconstant Z height with linear movement in both X and Y coordinates. Theultrasound probe 546 has a tip 548 that comprises a hydrophilic sphere.The modular ultrasound probe 546 is mounted to a CMM for movement, asdescribed above with reference to FIG. 3.

The XYZ position of the probe tip (i.e. the position of the centre ofthe hydrophilic sphere) in the coordinate system of the CMM is collectedvia the CMM controller at a high data rate (e.g. 1000-2000 Hz). This tipposition data is combined with a suitable signal generated by theultrasound probe at the same rate that indicates if any significantperturbation in the hydrophilic sphere's internal reflection echoes arepresent thereby indicating the sphere has made contact with an object.This signal, which is analogous to the trigger signal generated by atouch trigger probe, could be generated by monitoring the absolutedifference between the 2^(nd) internal reflection peak voltage (Vp)within each measured A-scan and a stored “no contact” reference A-scanas described above with reference to FIG. 15.

A touch event detected by the ultrasound probe thus causes the probe toissue an immediate instruction to the CMM (e.g. via a change in state ofa trigger signal line) which is used to stop CMM motion and store thepoint measurement. There will, however, always be some latencyassociated with delivery of the instruction from the probe to the CMMand also a period of CMM deceleration is unavoidable. The delay inhalting motion of the CMM causes the soft tip of the ultrasound probe todeform into the solid block so that its position when it comes to acomplete stop may be significantly away from the point P on the surfacewhere contact was first detected.

FIGS. 17(a) and 17(b) illustrate the above described effect. FIG. 17(a)shows the point P where contact is initially detected and FIG. 17(b)shows the further movement into the surface (i.e. to point O) thatoccurs before the ultrasound probe is brought to a halt. A closeapproximation to the point P could be made from interpolating thepositional and trigger signal data time series acquired up to thispoint. Alternatively, if the probe is travelling at a relatively highspeed when the touch event occurs, a more accurate result can beachieved with a back-off movement at a slower speed. This could involveremoving the probe from the surface in the opposite direction to theapproach vector but at a slower speed.

FIG. 18 shows plots of X-position 550, Y-position 552, Z-position 554and trigger signal (Vp) status data 556 as a function of time during amove of the probe into and then away from the surface in the mannerdescribed above. The time series plots show the time of contact 558 andthe time contact is lost 560 as dashed lines. The probe is thus movedinto contact with the surface and comes to a halt at point O. There isthen a short dwell period (which ends at point D) where the probe isstationary, before a slower back-off move is initiated. This slowerreverse move allows a higher density of spatial measurement points to berecorded and the temporal quantisation error from detecting the time atwhich Vp returns to the reference level thereby indicating the contacthas been broken has less influence upon the spatial quantisation errorin estimating the XYZ position of P (i.e. due to the shallowergradients). During the slower linear back-off movement, it would also bepossible within the probe hardware to generate the trigger signal (e.g.the Vp signal) from the A-scan data at a higher rate than the probeposition is reported. This would allow a more precise estimate of P tobe obtained by interpolation. More refined interpolation methods couldalso be employed that accommodate the subtle differences in reflectionpeak variations where contact deformation in the sphere occurs atdifferent locations on the spheres and at different grazing angles.

The elastic hydrophilic spheres of the above described coupling modulesmay be synthesised to release differing quantities of water during aninspection. Releasing higher quantities of water (e.g. for lubricationacross rougher surfaces) has been found to reduce the accuracy of anytouch contact measurements taken during a back-off move. This is becausea small droplet of water of varying size can accumulate around thelocation of the deformed sphere that causes a temporary physical bridgebetween the sphere and the inspection surface during the back-off move.This water droplet can introduce variability into when the ultrasounddata indicates contact with the object is lost. Such variability can beeasily overcome by not utilising the initial back off movement to takethe touch measurement, but instead incorporating a second movement intothe surface (e.g. along the same vector at the slower speed immediatelyafter the initial back-off movement) in order to take the touch positiondata.

The use of the ultrasound probe to also acquire surface contactmeasurements has the additional benefit of being quicker than exchangingthe ultrasound probe for a conventional surface contact (e.g. scanningor touch trigger) probe.

Referring to FIG. 19, it is also noted that due to the symmetry of thesphere only differences across latitudes (e.g. α and β) need to beconsidered. This is so long as the relationship between deformation andX Y or Z probe position of probe is linear or can be calibrated viaintentional surface contact touches.

In addition to analysing the A-scans of the ultrasound probe toestablish when surface contact is first attained, signal and dataprocessing algorithms for estimating the probe loading and thus couplingcondition during an inspection can also be included in the probe. Forsimplicity and because it is most relevant to how the probe maytypically be used, scenarios where the probe is loaded into theinspection surface from a nominally normal direction will now bedescribed. However, the same principles and methods can also be appliedwhen loading the probe at angles that depart from the direction ofL-wave travel (i.e. the axial direction of the transducer).

As already described with reference to FIG. 15, loading an ultrasoundprobe with a hydrophilic sphere tip against a surface induces measurablechanges to the internal reflection echoes within the A-scan that relateto the normal deformation or Z-displacement within the sphere. As alsodescribed above, monitoring the peak amplitude (Vp) and/or the time ofarrival (TOA) of the 1^(st) and 2^(nd) internal sphere echoes can thusbe used to assess loading. In some cases, it is noted that a singlecombined metric related to the internal reflection echoes can besensitive and robust enough for both contact detection and loadingdeformation estimation from calibration data (e.g. using a ratio ofhigher order reflection peaks).

FIGS. 20(a) and 20(b) illustrate an example of how the TOA and Vp,respectively of the first and second internal reflection peaks from thehydrophilic sphere of the ultrasound probe can change as the probe isgradually loaded normally on to the inspection surface at a constantslow speed (i.e. as the normal deformation of the hydrophilic sphere isgradually increased). It can be seen from the graphs of FIGS. 20(a) and20(b) that the relationship between the Z-deformation (or Z-deflection)in the soft sphere and the TOA and Vp of the 1^(st) and 2^(nd)reflection peaks are substantially linear.

This consistent and repeatable relationship between metrics defining theshape and/or position of the internal sphere reflection echoes withinthe A-scan and the amount of sphere deformation induced by probe loadingcan be usefully compiled during calibration. In other words, data asillustrated in the plots of FIGS. 20(a) and 20(b) can be generated byprobe loading measurements taken from an appropriate calibrationartefact; e.g. the artefact described above with reference to FIG. 13.Such known (i.e. by calibration) relationships can be used directlyduring any subsequent inspection to estimate the deformation or loadingcondition of the probe by extracting the same signal features from therelevant peaks (e.g. TOA, peak amplitude) within each measured A-scan.For example, a comprehensive set of calibration loading measurements canbe taken using the calibration block with the probe tip set at a rangeof set angles relative to the surface and a range of linear angles ofarrival on the surface (i.e. at different grazing angles). Such acomprehensive loading calibration is practical for a symmetrical probeas the compiled relationships between loading vector and the choseninternal reflection peak features is the same irrespective of theinitial axial rotation of the probe. Equipped with the set ofcalibration data related to deformation of the spheres, any new set ofreflection peaks from an A-scan measurement can be classified to inferthe deformation displacement either by interpolation (linear ornon-linear curve fitting) or simply a Euclidean nearest-neighbourclassifier.

It should be noted that the accuracy achieved in determining suchloading conditions may vary. The most robust and accurate estimations ofthe loading of the probe in terms of deformation of the soft conformaltip (in mm) will thus typically be achieved where the L-waves areprojected normally to the inspection surface along the probe axis.Fortunately, this is the most typical thickness measurement scenariowhere the part has parallel front and back walls.

In addition to acquiring individual measurements, continuous acousticscanning inspections are possible, for example, across simple geometriessuch as a continuous solid form with parallel front and back walls. Suchcontinuous scanning is preferably performed with an ultrasound probecomprising a hydrophilic elastomer sphere that is loaded against theinspection surface in a direction normal to the surface. Such continuousscanning is possible because of the self-lubricating action of thehydrophilic spheres and the ability to use loading estimations providedby analysis of the internal reflection echoes.

FIG. 21 depicts a scanning scenario in which a modular ultrasound probe600 having a tip comprising a hydrophilic sphere 602 tip is scannedacross an unknown, undulating inspection surface 604. The probecontinuously acquires A-scan measurements and from each A-scan itestimates the Z deformation (Zd) in microns. This can be performed at arelatively high rate so that any sudden changes in the loading conditionmay be immediately evident within the Zd time plot, as shown.

As indicated in FIG. 21, the probe is moved laterally across the surfacefrom a start point 606 to an end point 608. The probe is initially at aconstant height (i.e. a constant z-height) above the horizontal surfaceand is loaded at a constant level within the “coupling sweet spot”region that corresponds to a constant Z deformation (Zd) or constantsphere tip displacement. When the probe first reaches the undulatingregion 610 that has an increased Z height, the Zd estimation initiallyincreases without any deviation in the probe's Z-position. However, itis highlighted that the Zd estimation data may be used directly withinthe control loop of the CMM system to alter the probe height in responseto the measured change in Zd. As shown by the lower graph of FIG. 21,the CMM may be adapted to provide real-time adjustment of the probe'sheight (i.e. Z position) in response to the Zd measurement. In thisexample, this is done by moving the probe upwards in proportion to theincrease in Zd. This backing-off (in Z) of the probe thus results in theZd value quickly returning to its mean (optimum) loading condition.Similarly, when the probe reaches the reduced high surface, thereduction in Zd can be immediately compensated by the CMM in order tolower the probe back down towards the surface.

This technique thus uses the internal reflection echoes from within thesoft coupling layer of the hydrophilic sphere to directly, and in nearreal-time, ensure an optimal acoustic coupling condition. Such directreal-time estimation and thus control of probe loading conditions (i.e.using feedback control to the automation platform) against theinspection surface by interpreting the reflected L-waves within thecoupling module not only has benefits to probe positioning and scanning,but also fundamentally affects the transmitted L-wave from which usefulthickness measurements are made. It is noted that the highly elastic andconformable coupling elements also provide an inherent ability to varyand/or precisely control the projected L-wave beam entering the part byeither controlled variation in normal loading displacement orre-orientation of the axial probe vector away from the inspectionsurface normal, in accordance with both fundamental laws of acousticdiffraction and refraction respectively. This proactive beammanipulation is most practical and effective when the probe is attachedto a high precision automation platform, such as the CMM described withreference to FIG. 3.

FIGS. 22(a) to 22(c) illustrate some of the ways in which an ultrasoundprobe having a hydrophilic sphere tip can be used to induce more precisecontrol of the projected L-wave into objects. In particular, such aprobe allows more complex internal geometries to be probed withultrasound through either diffractive beam divergence control, by moreprecisely calibrated normal loading of the probe (i.e. varying theaperture size) or through refractive beam-steering control by precisionre-orientation of the transducer axis.

FIGS. 22(a) and 22(b) illustrate how increased loading of an ultrasoundprobe 620 with a hydrophilic sphere tip 622 can induce a wider diameteraperture on the inspection surface producing a collimating effect thatreduces the naturally divergent beam width. As shown in FIG. 22(b) thenarrower beam is beneficial since it avoids internal features 624 in theobject being inspected that could otherwise result in spuriousreflection echoes interfering with the measurements of interest.

Referring to FIG. 22(c), an object having non-parallel front and backwalls can be measured by re-orientation of the probe 620 away from thesurface normal. Such beam-steering may be limited to only smallrefraction angles to reduce mode conversion effects. For such smallangles the slower shear wave mode is less significant or it can betime-gated out of the A-scan.

A wide variety of variants to the ultrasound probes described above arepossible. For example, a plurality (e.g. 15-20) of hydrophilic spherescould be cascaded together to form a continuous chain of touchingspheres within a correspondingly long absorbent shell. The first spherecould be positioned within the shell so as to make contact with atransducer wear plate and the final sphere in the chain could protrudefrom the shell so as to contact the inspection surface. Such a probedesign would permit more remote inspection regimes where it isundesirable or physically impossible to position the transducer probetip close to the measurement node on the inspection part. The usefulapplication of such variant designs is possible because of the extremelylow L-wave attenuation properties observed in such hydrophilic mediacomponents. This results in negligible propagation loss from thetransducer to the coupling module tip.

In addition to such constructions offering an extremely efficientacoustic waveguide for the inspection L-wave, it is also possible tomanipulate the projected L-wave within a coupling module. Most notably,scenarios can arise where the L-wave inspection may need to be conductedalong some axis other than the normal probe axis (e.g. for measurementin confined spaces). For example, it would possible to embed an acousticreflecting mirror within a chain of hydrophilic sphere that simplyre-directs the L-waves in some known direction, in line with the laws ofacoustic reflection (i.e. angle of incidence equals the angle ofreflection). Such a reflecting mirror is simply a flat acousticallyreflecting surface (e.g. with a high acoustic impedance) mounted at aset angle.

FIGS. 23 to 25 illustrate a selection of different scenarios in which acascaded chain of hydrophilic elastomer spheres can be usefully applied.

FIG. 23 illustrates how a cascaded chain of hydrophilic elastomerspheres 640 can be used to inspect the bottom of a long and narrow hole.As indicated by the illustrated A-scan, it is noted that the primarymeasurement window for a probe comprising such a chain of spheresbecomes shifted in accordance with the number of spheres within thechain. However, the resulting back wall reflections within the A-scanare measureable and have a SNR approaching that achieved with a singlesphere.

FIG. 24 illustrates that the spheres can be sized so as to form atapered chain of spheres 650. The chain of spheres 660 can also beaccumulatively bent off-axis using weak diffractive effects. Again, theresulting back wall reflections within the A-scan are measurable andhave a SNR approaching that achieved with a single sphere.

FIG. 25 shows a chain of hydrophilic spheres 670 that comprise areflecting mirror 672 for inspections perpendicular to theprobe/transducer axis. Such an ultrasound probe can be useful forcomprehensive metrology inspection within tubes and/or containers wherethe probe can be rotated around the circumference of the enclosure.

Ultrasound probes may be provided with different (non-spherical) shapesof hydrophilic elastomer hydrated within a matching absorbent shell. Inaddition to the basic spherical shape described, super absorbentpolymers or lightly cross-linked vinyl elastomers with a high watercontent (e.g. typically 75-95%) can be synthesised so as to grow intopractically any closed-form continuous shape that is required whenhydrated, e.g. so as to fit perfectly inside the outer absorbent shell.A variety of bespoke (e.g. longer and/or thinner prismatic) shapes ofcontinuous hydrophilic elastomer material within matching PTFE shellscan be designed to accommodate any complex geometry part. By observingthe A-scan from such coupling elements, it is evident that the back wallreflections reside in the much wider first measurement window. It isalso noted that the various methods by which the internal reflectionechoes within the coupling module are processed to estimate loadingdisplacement or contact status also hold for such alternative designs.

As explained above, a compound class of coupling modules may be providedthat do not comprise a hydrophilic sphere. FIG. 26 illustrates theA-scans generated from an ultrasound probe with a compound class ofcoupling module, for example as described above with reference to FIG.7. As can be seen by comparing FIG. 26 with FIG. 15, there aredifferences in the signal that is generated with a compound class ofcoupling module and hence different processing can be used to interpretthe A-scans and extract information useful to the contact detection,loading, scanning and accurate thickness measurement processes.

FIG. 26 shows an ultrasound probe 700 with a normal beam compoundcoupling module comprising a latex rubber tip 702 loaded on to a simpleinspection surface.

FIG. 26(a) shows the probe 700 approaching a surface. Prior to makingcontact with the surface, it can be seen that the A-scans incorporatesonly evenly spaced repeated reflections (i.e. first, second and thirddelay line reflection peaks 701, 703 and 704) from the rigid plasticdelay line element 704, after the initial Tx pulse (not shown in thefigure). These reflections show negligible measurement to measurementvariability whilst the probe moves through free space within the CMMvolume.

Referring to FIG. 26(b), when the latex rubber probe tip 702 comes intocontact with the surface, there is an immediate variation in themeasured A-scan response.

This initial tangential contact induces no visible shift in the time ofarrival of the internal reflection echoes, yet induces quite an obviousreduction in the 2nd and 3rd reflection echo amplitudes (i.e. peaks 710and 712). Depending on the coupling performance (e.g. defined by surfacefinish soft coupling and the part geometry), this reduction in reflectedenergy occurs in conjunction with increased back wall reflectionwaveforms 714 from energy transmitted into the part.

Referring to FIG. 26(c), as the probe 700 is loaded further on to thesurface the soft coupling layer (i.e. the latex rubber tip) deforms sothe rigid planar delay element comes into closer conformal contact withthe surface. The amplitude of the 2nd and 3rd reflection echoes (i.e.peaks 720 and 722) reduce further and there is an increase in theamplitude of the successive back wall reflections (i.e. peaks 724).However, this reduction in peak amplitude for the delay line peaksignals (i.e. peaks 720 and 722) is not accompanied by any change in thetime of arrival (TOA) or phase of these reflection peaks. Moreover, ashighlighted by FIG. 26(d), withdrawing the probe entirely from thesurface causes the delay line peaks to return to the same levels andadopt the same shape as before the surface contact was made.

FIG. 27 depicts how the time of arrival and the peak amplitude of thefirst and second delay line reflection peaks for the probe describedwith reference to FIG. 26 evolve as the probe is loaded linearly on tothe inspection surface. In particular, FIG. 27 shows that the reflectionpeaks from the rigid delay medium remain fixed in time relative to theT=0 excitation within the A-scans. Loading calibration is thus moreeffective using waveform features that quantify the proportion ofacoustic energy that can leak from the rigid delay medium with increasedloading condition (e.g. ratios of 2nd or 3rd order peak voltages Vp).However, it is emphasised that such plots can still be usefully compiledduring a calibration procedure and used to automatically detect contacts(e.g. through a hard threshold energy detector) or classify loadingconditions (e.g. through linear or polynomial interpolation from theplots or some other computationally efficient classifier).

A signal processing method for generating thickness measurements fromthe measured A-scans within the measurement window will now bedescribed. Any such signal processing algorithm is preferably robust andmay be based upon a form of generalised cross correlation or replicacorrelation to extract accurate time delays between first, second andpossibly third back wall reflections. It should be noted that beyond thethird back wall reflection, waveform dispersion can start to affect thetime difference estimation accuracy due to perturbations in thefine-scale shape of the return.

A preferred signal processing approach employs a form of replicacorrelation processing. This technique allows robust, computationallyefficient and accurate time-delay estimation. In particular, across-correlation algorithm with spectral pre-whitening retains accuracybetter than conventional amplitude threshold time of arrival methods.Although a replica correlation process is preferred, it should be notedthat other techniques could be used. For example, a square-law amplitudethreshold detector could be used in which reflection peaks are assumedto be detected at the point at which the waveform intensity or amplitudeexceeds some set threshold. One dimensional edge detectors or waveletdecomposition techniques could also be used as they allow the requiredtemporal accuracy to be maintained whilst smoothing noise. However,cross-correlation algorithms are better suited to high-frequencyreal-time implementation.

Referring to FIG. 28, the function of a replica correlator isschematically illustrated in which the absolute time delay betweensuccessive back wall reflections can be estimated with an accuracy thatapproaches the fundamental measurement resolution provided by thedigital acquisition system (e.g. a time equating to the reciprocal ofthe ADC sample rate). A replica correlator is a form of matched filterwhere the output is computed from the cross-correlation between themeasured A-scan and a delayed replica of a backwall reflection.

In particular, FIG. 28 illustrates how the replica correlation processinvolves correlating the time-windowed A-scan response y(n) with one ora bank of stored or extracted back wall reflection waveforms x(n). Thiscorrelation process is implemented by transforming the input waveformsx(n) and y(n) to the frequency domain using first and second DFTalgorithms 750 and 752 respectively. The first and second DFT algorithmalgorithms 750 and 752 may comprise any suitable form of the well knowFFT algorithm. A multiplicator 754 then performed successivemultiplication operations in the frequency domain on the transformedsignals. The output of the multiplicator 754 is converted back into thetime domain by a third DFT algorithm 758 and a peak detector 760 outputsdata to a delay estimator 762. When using the favoured phase transformversion of the generalised cross-correlation (GCC-PHAT) algorithm, onlythe signal phase information is preserved after the cross-spectrum isdivided by its magnitude. Ideally, with no additive noise, thisprocessor approaches a delta function centred at an accurate time delayestimation.

Such a fast convolution process may suffer from inaccuracy whentransforming the result back to the time domain due to spectral smearingand leakage. Therefore, a pre-whitening filter 756 is implemented inorder to improve the temporal accuracy and SNR robustness of the timedelay estimation process. Phase transform pre-whitening has the effectof equalising the cross spectra (Pxy) phase so as to maximise the SNRand temporal accuracy of the dominant delay against multipathreverberation. Although it can be most effective form of pre-whiteningfor A-scans measured with the probe, any such correlation method (e.g.Knapp and Carter) can be used. For example, the phase transform methodmay become less effective for low SNR environments.

Referring next to FIG. 29, the principle of the phase transform replicacorrelator algorithm is illustrated. In particular, this figureillustrates a measured back wall response from the measurement windowbeing presented to the signal processing stage.

The repeated back wall reflections (i.e. peaks 780) within themeasurement window exhibit strong correlation in terms of repeatedshape, especially in terms of their phase. Although the signal levelattenuates from the first reflection echo 780 to the third reflectionecho 782, the SNR is still relatively high. As indicated, themeasurement window response can be correlated with a stored replica ofthe back wall reflection echo using the phase transform version of thecross correlator (GCC-PHAT). This generates a correlation response 784with some noise suppression (i.e. the correlation process induces someSNR gain) and effective sharpening of the waveform representing eachecho at the exact time sample with a maximum phase correlation. Fromthis, a simple maximum peak detector can determine the time sample ofeach echo and the time difference between these peaks (t1 and t2)represents the time delay from which the thickness estimations can bemade. Although the replica waveform can be measured during calibration,it is also indicated that the same algorithm can be effective byextracting the back wall waveforms directly from the A-scan itself or byan auto-correlation version of the algorithm.

In general, the data processing employed to assess every measured inputA-scan in order to simply detect tip contacts (e.g. by a hard thresholddetector) and/or to quantify the deformation in the soft coupling module(e.g. Zd) in order to ensure optimal coupling requires less computationthat the subsequent signal processing required to measure the thicknessof the coupled part. Therefore, a practical feature of the probe is thatit may function in more than one automated mode of operation.

FIG. 30 depicts a flow chart showing possible modes of operation,control and flow of data between the probe and the peripheral hardware.As shown, a probe movement command can be induced from the probe controlsoftware through the CMM/head controller with ultrasound A-scans beinggenerated simultaneously at a high rate. As a matter of course, basicprocessing could extract ultrasound reflection peak features for everymeasured A-scan and assess, via a hard threshold detector, whether thetip is in contact with any object. If no contact is detected, themovement induced by the probe controller through the CMM/head controllercan continue with more A-scans being acquired, but no other processingbeing executed on the probe.

It should be noted that the measurement resolution of the probe orapparatus when applying spectral correlation methods (e.g. the GCC) todetermine the accurate time delay information from which the thicknessof the contacting part can be directly estimated is typically limited bythe digitising sampling frequency within the receiver electronics (i.e.the ADC). In situations where the reflection waveforms of interest aresmooth predictable high SNR signals without significant stochastictransient components (e.g. bandlimited), it has been found to bepossible to artificially increase the effective system resolution byregular interpolative up-sampling of the raw measured A-scan responseand the replicas prior to applying the correlator. Moreover, dependingupon the SNR and noise statistics of the measured A-scans, a selectionof different pre-whitening filters (i.e. weighting functions in thefrequency domain) can be applied within the signal processing to makemore accurate thickness measurements. Such pre-whitening filters includethe smoothed coherence transform, the Roth filter or the Hannah andThompson filter.

It is also noted that the Phase Transform version of the GCC algorithmthat is mentioned above may not always be optimal for lower SNR regimes,when used in isolation. For example, A-scans measured with the apparatuscontaining the back wall reflection and or internal delay linereflection signal of interest could also contain higher levels ofelectronic noise from the measurement system (e.g. in cases where noaveraging is applied across A-scans or lower cost instrumentation isemployed) or higher acoustic background noise within the sensing systemmeasurement band (e.g. when measurements are taken using high acousticnoise assets or a high noise automation platform such as the crawlerplatform described below). The skilled person would thus appreciate howthe most appropriate signal analysis technique may vary for differentapplications.

If contact is detected, the CMM/head controller can be immediatelyalerted, so as to report probe position at contact and/or induce someinterrupt movement action. This contact detection could alsoautomatically enable the probe to begin processing the ultrasoundreflection echoes to quantify the ultrasound probe coupling deformationagainst the inspection surface (i.e. related to the “coupling sweetspot”). More A-scans could continue to be generated at a very high rate,still without any computationally intensive thickness measurement signalprocessing being activated, until the coupling contact (i.e. couplingZ-deformation) is deemed to be within the “coupling sweet spot” or thecoupling deformation tolerance.

Although this acceptable coupling condition would be determinedprimarily by the methods described, since it relates mostly toassessment of the soft coupling element (e.g. hydrophilic sphere), itcould possibly include some basic assessment of back wall reflections inthe measurement window (e.g. simple metrics such as kurtosis or peakamplitude etc).

Following automated qualification that the probe coupling is sufficientor optimised, the more computationally and time intensive signalprocessing algorithm within the probe could be activated to extract thesuccessive back wall reflections within the primary measurement windowof the A-scans and measure their time delay. For many scenarios, e.g.continuous scanning across a part, it may in practice be necessary forthe probe to continuously execute the thickness measurement processingalgorithm for every measured A-scan. In this way, the computationallyefficient yet accurate time delay estimation method provides importantoperational benefits.

FIG. 30 thus represents one architecture for the probe in which a largeproportion of the processing is accomplished by processing units (e.g.FPGA, DSP, CISP) within the ultrasound probe itself. This distributedprocessing architecture has advantages for deployment upon a CMM wherecommunication channel bandwidths through the measurement head and scopefor local memory storage on the probe can be limited. However, it isnoted that there are other possible architectures and methods forprocessing the A-scan data. For example, in some cases, it may bepossible to affect all of the rudimentary processing for contactdetection and optimal coupling “on the fly” but record extended batchesof A-scans locally in memory, for batch transfer and post-processing ofthe thickness measurement on some other processor (e.g. a laptop or PC).Regardless of the processing architecture, the basic signal processingindicated here to estimate the time delays via mode-3 gauging remainboth robust and accurate.

Embodiments of the ultrasound systems described herein and theassociated automated inspection concepts may be useful for internalmetrology measurements within highly attenuating and much thickermetallic and non metallic parts than those described above. Inparticular, the ultrasound apparatus may be used to measure certain highvalue safety critical metallic parts (e.g. medical implants) that havebeen manufactured using additive manufacturing (AM) methods (e.g. usinga selective laser melting machine). Such parts may require both internalmetrology measurements and non-destructive gauging to ensure that theporosity exhibited across the part is within the required tolerance.This type of porosity measurement is becoming more important as such AMtechniques can specifically design-in a variety of porosities across thepart. It is highlighted that such porosity distribution estimationacross an AM solid part of known or measured geometry may be adequatelyachieved by direct estimation of the sound speed distribution across thepart. This is because a strongly linear and easily calibratedrelationship exists between longitudinal sound speed and porosity. Alinear relationship exists between the shear wave velocity and theporosity also. This could also be used in some circumstances.

The modular ultrasound apparatus described herein is also capable offocussing projected ultrasound at known depths or angles within a part(e.g. using a coupling module having a spherically focussedplanar-concave lens). The apparatus could thus be used for the automateddetection, location and sizing of internal defects and voids withininspection parts.

The modular ultrasound probe mentioned above may advantageously be usedfor dimensional form measurements of soft material parts. For example,some softer solid gelatinous, organic or non-metallic parts may not beinherently suited to conventional contact probing as the surfaceinteraction into such softer parts may not cause the probe stylus tomechanically deflect in a consistent manner (e.g. due to mechanicalhysteresis effects) and/or such contacts could conceivable induceunwanted indentations in the soft parts under inspection. Such partscould include soft elastic or plastic polymer membranes, fabrics andleather, food stuffs and even organic membranes and human tissues.

Such soft metrology tasks could involve the soft coupling element withinthe probe being specifically selected to be softer than the object ormaterial being inspected. This is so the coupling element deflects by ameasurable amount before inducing any deflection within the part beinginspected. An extremely soft hydrophilic vinyl polymer with a very highwater content (e.g. 95%) could thus be provided as the coupling elementof an ultrasound probe. Such a coupling module could be provided as oneof a set of coupling modules.

The modular ultrasound probe mentioned above may advantageously be usedfor dimensional form measurements of high quality surface finish parts.In particular, it is further noted that some other precisionmanufactured rigid parts with complex geometries but with an extremelyhigh quality smooth polished surface finish may require automateddimensional measurements. However, contact interaction between a hardruby ball stylus probe and such polished inspection surfaces may not beideal as such interactions could potentially induce some scratches orimpact damage on the surface. Moreover, alternative non contact opticalmeasurement probes (e.g. a laser scanning probe) may not be suitable dueto the non conducive optical properties of the inspection surface (e.g.an optically transparent acoustic concave or convex lens or a parabolicoptical mirror).

The modularity of the ultrasonic inspection apparatus described aboveand more specifically the inherent ability to automatically change andtailor the different coupling module designs to accommodate specificinspection conditions is particularly beneficial during the measurementof complex geometry parts. Moreover, the choice of any specific couplingmodule will often dictate the specific gauging methods employed togenerate output measurements (e.g. thickness values across eachmeasurement node).

For example, during the inspection of a typical hollow aerospace blade,a selection of different coupling module designs and associated gaugingmethods could be employed in succession for more comprehensive coverageinspections. Specifically, a large proportion of the part's bulkexternal surface may be parallel to the internal back wall surface. Forsuch bulk “skin thickness” gauging, coupling modules that comprise ahydrophilic sphere may be attached to the ultrasonic probe andcontinuously scanned laterally across the blade with the probe retaininga substantially normal orientation to the inspection surface. Suchnormal incidence continuous scanning without the probe leaving theinspection surface utilises the self-lubricating property of this typeof soft conformal and elastic hydrophilic coupling module, as previouslydescribed. It therefore facilitates a very high density of measurementpoints across the inspection surface via the mode-3 gauging method usingthe above described replica correlation method for robust time delayestimation at each node.

However, it is noted that the method for measuring bulk skin thicknessis not necessarily appropriate for inspection across the entire blade.For example, in the vicinity of the leading and trailing edge of theblade aerofoil, the external front wall and internal back wall oftendepart from such a parallel geometry. In this instance, a refractivecoupling module may be used to project ultrasonic L-waves in therequired direction towards the internal back wall. This could involveusing a coupling module having a fixed rigid delay line, with anappropriate refractive wedge angle. Alternatively, with appropriatecalibration, the refractive inspection task could be accomplished usinga coupling module having a hydrophilic sphere but with the probeorientated at the appropriate angle from the surface normal. In eithercase, mode-3 inspection becomes problematical because the non-parallelfront and back wall surfaces prevent successive back wall reflectionsreturning to the probe. Mode-2 gauging in which the time delay betweenthe first delay line or internal reflection peak from the couplingmodule and the back wall is estimated, is also not appropriate as nostrong internal reflection peak would exist at such a refractive angle.Therefore, in this case, mode-1 gauging (in which the absolute timedelay between the initial excitation pulse and the subsequent back wallreflections is estimated) could advantageously be implemented instead.

To obtain the highest possible mode-1 thickness gauging accuracy, afurther calibration procedure may be required in which a range ofrefractive measurements are made across a refractive calibration blockmachined from the same material as the inspection part and whichincorporates one or more back walls at the same front wall and back wallorientation as the inspection part. In the same way as any such mode-1calibration procedure, use of the same material in the calibration blockas the inspection part allows the sound speed calibration to beintegrated into this refractive angle calibration. That is, a separatesound speed calibration procedure can become unnecessary because a rangeof backwall time delays for known refractive thicknesses taken duringcalibration can mean that any further time delay measured with the probeduring the inspection can directly infer the unknown thickness by alinear interpolation.

It is also possible to determine the speed of sound within the softcoupling element of a coupling module. Although such a sound speedmeasurement is by no means essential (e.g. it is not required foraccurate thickness inspections of parallel eccentric parts such asairframe skins and parallel hollow blades using the mode-3 method), itdoes have some advantages. For example, the sound speed of the couplinglayer within the attached coupling module may be affected slightly byatmospheric temperature variations and, for certain probe functions suchas off-normal axis inspections of non-parallel front and back wallinspection parts, it can be beneficial to calibrate (i.e. measure) thesound speed of individual coupling modules within the inspection. Morespecifically, the sound speed of the coupling medium will directlyaffect the angle of projection of the ultrasound waveform into theinspection part, in accordance with Snell's Law of Refraction. Hence, amore accurate and calibrated measure of this absolute sound speed withinthe homogeneous isotropic coupling medium can be beneficial for buildingup and projecting the exact position and orientation of internalreflection surfaces.

The measurement of a coupling element's sound speed can also havealternative applications; e.g. classifying unknown liquid samples thatinteract with the coupling element. The speed of sound of the couplingmodule (CL) could be derived by direct measurement of the longitudinaldimensions (d) of the coupling medium and estimation of the roundtriptime of flight (t) (i.e. using the relationship CL=2*d/t) via mode-1mode-2 or mode-3 methods. Alternatively, a method could be used thatinvolves linearly loading the probe in a highly controlled and precisefashion on to a known planar surface. More specifically, it has beenfound that measurement of the change in time of arrival (TOA) of thefirst internal reflection waveforms from the coupling layer as the probeis loaded on to a planar surface allows the linear relationship betweenthis TOA and the coupling layer deformation (i.e. the linear loadingdisplacement) to be compiled. From this linear relationship plot (i.e.for a incompressible coupling layer), the sound speed can be calculateddirectly as the gradient. In other words, the absolute gradient betweenthe TOA(t) Vs Z-deformation or loading (r) equates to half the soundspeed (CL) from the relation r=(CL*t)/2. This method has been found tobe accurate and ensures that no potentially inaccurate estimation of theexact heuristic longitudinal dimension of the coupling layer is requiredto calculate the coupling layer sound speed.

Although it is described above how the ultrasound apparatus can beinstalled on a bridge type CMM, it should be noted that it can be usedwith other apparatus.

FIG. 31 illustrates how the ultrasound probe 802 described above can bemounted on an x-y scanner 800.

FIG. 32 illustrates how the ultrasound probe 802 described above can bemounted on an ultrasonic crawler system 810 for measuring internalcracks and corrosion within thin aerospace structures (e.g. the fuselageskin). In such an embodiment, strong reflection echoes that occur in theA-scan between the back wall reflections can be detected as additionalunwanted interfaces within the part volume that may be classified as aninternal void or crack.

In such an embodiment, the crawler vehicle 810 may implement continuousscanning over a large curved structure to measure the internal thicknessof the structure's skin (e.g. an aerospace structure or wind turbineblade). However, the curvature of the part and its interaction with thecrawler wheels may induce some variability in the exact clearancebetween the mobile platform upon which the ultrasonic probe is mountedand the inspection surface. By high resolution estimates of the currentdeflection (Zd) in the soft coupling tip, it is possible to adapt the Zposition of the probe relative to the platform as it travels along thepart to compensate for the variable clearance, thus retaining the probedeformation within a set tolerance (i.e. the ‘coupling sweet spot’) . Inits simplest form, adaption of the probe height relative to the platformin response to variation in the soft coupling element deformation couldbe implemented using a linear stage motor with a simple linear encoderattached to the platform that would allow the Z-height of the probe tobe varied in real-time during the inspection. In a more sophisticatedembodiment, a second rotational motor and encoder may be incorporatedallowing the probe to be rotated whilst remaining in the plane of itsmotion so as to alter its angle of incidence against the surface inresponse to changes in the surface normal.

1. An ultrasound probe comprising; a transducer for transmitting andreceiving ultrasound, and a coupling element for contacting andacoustically coupling to an object to be inspected, wherein theultrasound probe comprises an analyser arranged to analyse theultrasound signal received by the transducer and thereby determine ifthere is contact between the coupling element and the surface of anobject.
 2. An ultrasound probe according to claim 1, wherein thetransducer is arranged to periodically transmit an excitation pulse andsubsequently receive the returned ultrasound signal, the amplitude ofthe ultrasound signal received after each excitation pulse beingmeasured as a function of time to provide an amplitude scan.
 3. Anultrasound probe according to claim 2, wherein each amplitude scancomprises amplitude peaks relating to internal reflections of ultrasoundfrom within the probe, the analyser determining if there is contactbetween the coupling element and the object from changes in theamplitudes of the internal reflections between successive amplitudescans.
 4. An ultrasound probe according to claim 2, wherein the analyserdetermines if there is contact between the coupling element and theobject by assessing if internal reflections from a contacted object arepresent in each amplitude scan.
 5. An ultrasound probe according toclaim 1, wherein the ultrasound probe outputs a trigger signal thatindicates when the analyser has determined the coupling element is incontact with an object.
 6. An ultrasound probe according to claim 1,wherein the analyser determines when optimum ultrasound coupling with anobject is achieved by monitoring internal ultrasound reflections fromthe contacted object as the coupling element is moved into closerengagement with the object.
 7. An ultrasound probe according to claim 1,wherein the analyser determines the thickness of a contacted object frominternal ultrasound reflections from the contacted object.
 8. Anultrasound probe according to claim 7, wherein the analyser assesses thethickness of a contacted object from successive reflections from theback wall of the object.
 9. An ultrasound probe according to claim 1,wherein the coupling element comprising an elastically deformablematerial.
 10. An ultrasound probe according to claim 1, wherein thecoupling element is substantially spherical.
 11. An ultrasound probeaccording to claim 1, wherein the coupling element comprising aself-lubricating material.
 12. An ultrasound probe according to claim11, wherein the self-lubricating material comprises a hydrophilicelastomer sphere.
 13. An ultrasound probe according to claim 1,comprising a delay line.
 14. An ultrasound probe according to claim 1,comprising a base module that comprises the transducer and a couplingmodule that comprises the coupling element, wherein the base modulecomprises a first connector portion and the coupling module comprises asecond connector portion, the first connector portion being releasablyattachable to the second connector portion.
 15. A coordinate positioningapparatus, comprising an ultrasound probe according to claim
 1. 16. Amethod of operating a pulse-echo ultrasound probe mounted to coordinatepositioning apparatus to acquire surface position measurements, themethod comprising the step of monitoring the echoes of receivedultrasound signals for changes indicative of contact with an object.